I dunno, I occasionally buy rules "just to read", and occasionally buy models "just to paint".
Not very often though, and less so with rules.
With models, I tell myself that it will be a fun one-off little distraction (like I need more of those...

).
With rules, I actually find them interesting from a mechanical sense. That is, I like to see how different ideas about manoeuvring, action, and combat are all conveyed through different rules. As I regard games as being like puzzles that need to be solved, seeing other's solutions is always quite enlightening.