This is related to another question of mine.  Suppose you met someone who was well-acquainted with the basic properties of rings, but who had never heard of a module.  You tell him that modules generalize ideals and quotients, but he remains unimpressed.  How do you convince him that studying modules of a ring is a good way to understand that ring?  (In other words, why does one have to work "external" to the ring?)  Your answer should also explain why it is a good idea to study a group by studying its representations.
In short, I'd tell your friend:  "If you believe a ring can be understood geometrically as functions its spectrum, then modules help you by providing more functions with which to measure and characterize its spectrum."

Elements of a module over a ring $R$ are like generalized functions on $Spec(R)$.  We can talk about the support of a module element, or its vanishing set.  More concretely, think of how global sections of a line bundle can act as functions you can use to define map into projective space.  

When you glue together a module on open sets of a spectrum or a scheme, you get to glue using maps which are module isomorphisms, which are more flexible than the ring isomorphisms required to glue together a scheme.  Borrowing intuition from smooth manifold land, the twist in the Moebius band (as a line bundle on the circle) is formed by gluing a copy of the reals to itself via multiplication by $-1$, a module map, not a ring map.  This allows us to think of functions like $\cos(\theta/2)$ as being globally defined: as a map to the Moebius band. 

In the same vein, when you have a representation $V$ of a group $G$, each element $v\in V$ gives you a nice evaluation map from $G$ into $V$, so lurking everywhere we've got these morphisms from our object of interest into a known object, which are nicely related to each other via the group laws.  A fortiori, this certainly doesn't capture the full utility of group representations, but a priori I think it's a decent justification.
