I've always thought sub-q takes longer for your "system" to absorb.
IM, is for quicker absorbption into your bady as well as any "localized" effects - such as from no-ester drugs, increased IGF-1 activity at testosterone site, ets.
I remember bigkiwi like to do insulin IM cause it would react much quicker.
I don't use IGF-1 LR3 but would use it IM for localized growth benefits - why would you not.
I'm sure there's some thing that definately you wouldn't want to do IM vr sub-q and vice-versa. Like GH - it's absorbed rather quickly in the body and may get in too quickly if done sub-q. There's some Synthek products that can be used either way and one way will irritate the site more than the other.