TITLE: Abel's equation for the dilog
QUESTION [16 upvotes]: Abel's identity for the dilogarithm (see the  wikipedia page about polylogarithms)
plays a role in web geometry as it is one of the abelian relations of the
first example of exceptional web (Bol's 5-web) to appear in the literature. 
I have heard it is important in other domains (cohomology of SL(3,C), algebraic K-theory, motives ). 
I would like to learn more about it. 
I am asking for:

Insights on why Abel's identity is relevant in this or that field;
References where it plays a role.


Edit.
I have just learned from this blog about Bridgeman's orthospectrum identity.
Those interest in the question above might want to take a look at it.

REPLY [8 votes]: There is a remarkable article, "The remarkable dilogarithm," J. Math. Phys. Sci. 22 (1988),   131--145, by Don Zagier, which was recently reprinted and updated as "The dilogarithm function" (63 pages!) in one of the collections by Springer Verlag.