TITLE: The category of elements, enrichment, and weighted limits
QUESTION [13 upvotes]: This is a crosspost of this MSE question.

Every so often, when reading notes online or skimming through books, the category of elements and the Grothendieck construction pop up. I don't know anything about the Grothendieck construction, and I don't understand the significance of its special case - the category of elements. This is probably going to be the first in a series of questions about the Grothendieck construction (I'll interlink with later ones if/when I ask them).
My main motivation for asking this question is the excerpt below from Borceux vol II, sec 6.6. I have tried to go into detail about how I don't understand the significance of the category of elements in this context.

The first observation for enriching the notion of limit is the fact in many fundamental results of the previous chapters, we had to deal with a situation like in diagram 6.3.6, where $F,G$ are functors and $\mathsf{Elts}(G)$ is the category of elements of $G$; the limit considered was that of $F\circ \phi _G$ [...]. Restricting one's attention to limits of the form $\varprojlim (F\circ \phi _G)$ is not at all a restriction since, choosing for $G$ the constant functor on the singleton, the category of elements of $G$ is just $\mathscr A$ itself and $\phi _G =1_\mathscr{A}$, so that we recapture the limit of $F$.

But the key observation for enriching the notion of limit is the following fact
Lemma 6.6.1 In the situation which has just been described, there exist bijections natural in $B\in \mathscr B$,
  $$\mathsf{Nat}(\varDelta_B,F\circ \phi _G)\cong \mathsf{Nat}(G,\mathscr B(B,F-))$$ where $\varDelta _B:\mathsf{Elts}(G)\longrightarrow \mathscr B$ is the constant functor on the object $B\in \mathscr B$.

From this lemma Borceux concludes that $\varprojlim (F\circ \phi_G)$ exists iff $\exists L\in \mathscr B$ with bijections natural in $B$:
$$\mathsf{Nat}(G,\mathscr B(B,F-))\cong \mathscr B(B,L)$$
And from this, he almost has to write the definition of a weighted limit.

Alternatively, here's an approach I find somewhat more intuitive, but one that does not explicitly mention the category of elements at all.
In classical CT, we define limits using $\varDelta$, which is not available in a general enriched category $\mathsf C$ since $\mathsf C(A,B)$ has no elements to work with. In the unenriched case, one could also try to define $\varDelta$ via the cartesian closed structure of $\mathsf{Cat}$ as a right adjoint of the projection $\pi:\mathsf C\times \mathsf J\longrightarrow \mathsf C$, but this doesn't generalize either since most enriched categories are not cartesian closed. So we're left with trying the representability approach - a limit of $F$ is just a representation of $\mathsf{Nat}(\varDelta-,F)$. Here we can escape the "problems of enrichment" because I think the following holds:
Proposition There's a bijection natural in $X$: $\mathsf{Nat}(\varDelta \ast,\mathsf{Hom}(X,F-))\cong \mathsf{Nat}(\varDelta X,F)$, where $\ast$ is the singleton.
The point is to now only mention the functor $\varDelta \ast$ instead of the general $\varDelta$.
Digression - is the above bijection also natural in $F$? (I think it is.)
Now since $\varprojlim F$ is a representation of $\mathsf{Nat}(\varDelta -,F)$, we can define it as an object with bijections natural in $X$$$\mathsf{Hom}(X,\varprojlim F) \cong \mathsf{Nat}(\varDelta \ast,\mathsf{Hom}(X,F-))$$
Now we have reduced the usage of $\varDelta$ to the functor $\varDelta \ast$ alone. The role of $\varDelta \ast$ as a weight is also easily seen, as each component of natural transformation $\eta _A :\varDelta \ast \Rightarrow \mathsf{Hom}(X,F-)$ picks out an arrow $X\rightarrow FA$, and so replacing 
$\varDelta \ast$ by say, $\varDelta (\ast\coprod \ast)$ would give a "double cone". Replacing $\varDelta \ast$ by more general functors $G$ would produce more intricate cones.
This alternative approach is (I think) completely ignorant of the category of elements, and yet manages to give a nice motivation for the definition of a weighted limit. On the other hand, Borceux's approach just poops it out by category-of-elements-magic.

What exactly is happening here? What is the role of the category of elements in this story and why does it effortlessly yield the definition of a weighted limit?

REPLY [6 votes]: The question is a bit imprecise, so I'm not quite sure whether this will really answer it, but let me try to paint the reduction of weighted colimits to conical colimits in a more "conceptual" light.
First of all, your identity $[\mathcal{A},\mathsf{Set}](\Delta \ast, \mathcal{B}(X,F)) \cong [\mathcal{A},\mathcal{B}](\Delta X, F)$ is correct; we can write it out in terms of ends:
$\int_{A \in \mathcal{A}} [\Delta \ast(A), \mathcal{B}(X,F(A))] = \int_{A \in \mathcal{A}} \mathcal{B}(X,F(A)) = \int_{A \in \mathcal{A}} \mathcal{B}(\Delta X(A), F(A))$
Now, let me argue that the key identity is $G = \operatorname{Lan}_{\phi_G} (\Delta \ast)$. This identity implies that the $G$-weighted limit of $F$ is the $\Delta \ast$-weighted limit of $F \phi_G$, because we have
$[\mathcal{A},\mathsf{Set}](G, \mathcal{B}(X,F)) = [\mathcal{A},\mathsf{Set}](\operatorname{Lan}_{\phi_G} (\Delta \ast), \mathcal{B}(X,F)) = [\mathcal{A},\mathsf{Set}](\Delta \ast, \mathcal{B}(X,F\phi_G))$
That is, the defining property of a left Kan extension relates the functor represented by a weighted limit, on the left, to the functor represented by the conical limit, on the right.
So from this perspective, the key property of the category of elements is that it allows us to express our copresheaf as the left Kan extension of a constant-at-1 presheaf. I'm not sure whether this is a universal property, but it is reminiscent of the universal property of a subobject classifier, and of a universal property the category of elements does possess -- it is the classifying discrete opfibration. We have a diagram:
$\require{AMScd}$
\begin{CD}
    \mathsf{el}G @>>> \ast \\
    @V \phi_G V V @VV \ast V\\
    \mathcal{A} @>>G> \mathsf{Set}
    \end{CD}
The property I'm pointing out is that the bottom is a left Kan extension, while the usual universal property says that this is a pullback comma square. I'm not quite sure of the relationship.
EDIT
Just an additional miscellaneous thought: if this were also true in the enriched setting, then this would tell us for example that every group representation was the induced representation of a trivial representation of some algebra (or rather, algebroid -- and of course, "trivial representation" doesn't even make sense in this generality). This isn't true, but it's still a good strategy to look for representations as subrepresentations of induced representations of trivial representations.