I would motivate the simple loop conjecture as follows.  (I'm fairly idiosyncratic about this; I fear I'm going to turn off many 3-manifold topologists.)

As well as understanding spaces, we want to understand the maps between them. One instance of this is that it would be extremely useful to have some sort of 'classification' of the set of *all* maps from your favourite (closed, say) surface $\Sigma$ to *any* 3-manifolds

What might such an understanding look like?  If we replace 3-manifolds by graphs, then the answer is provided by a folklore theorem, often attributed to either Stallings or Zieschang.

>**Folklore theorem:** Every continuous map from $\Sigma$ to a graph $\Gamma$ kills an essential simple closed curve.

This fits into Sela's framework of *Makanin--Razborov diagrams* (over free groups), where it implies that the natural homomorphism $\pi_1\Sigma\to F$ induced by including $\Sigma$ in the boundary of a handlebody forms the Makanin--Razborov diagram for $\pi_1\Sigma$.

So the simple loop conjecture is the analogous statement over 3-manifolds. Basically, it would take us from having a relatively hazy understanding of what the set of *all* maps from $\Sigma$ to a 3-manifold might look like, to a fairly complete understanding.

You might say "Fine, but what's so special about surfaces?"  In fact, surfaces play a distinguished role, because of the way they arise naturally in JSJ theory.  For this reason, they are one of the key cases; if we can understand maps from surfaces to 3-manifolds, we have a chance of understanding maps from *arbitrary* aspherical spaces to 3-manifolds.