TITLE: Why study infinite loop spaces?
QUESTION [10 upvotes]: What makes an infinite loop space an interesting object of study for homotopy theorists? The reason I ask this question is that I found a lot of results treating the question of whether a given space is an infinite loop space. So it seems that the property of a space being homotopy equivalent to an infinite loop space opens totally new possibilities and techniques to study the space. I would glad if anybody could take some time to give an overview about the most well-known directions one can study a space recognized as infinite loop space.

REPLY [3 votes]: There is so much much more.  For one historical starting point among many, you see that many spaces of interest are infinite loop spaces and that tells you how to calculate things about them.  For just one example, to add to David's parenthetical clause, almost everything we know about characteristic classes for topological bundles comes from the infinite loop structure of BTop.  This is very concrete and calculational and tells us geometrically about topological cobordism.  At another extreme, knowing that algebraic K-theory is given by E infty ring spectra is the starting point for derived algebraic geometry.   I could go on for pages and pages.  The emerging equivariant story is even richer and promises far more to come.