Theoretical justification for limping

[William Jockusch]

Suppose I am playing some form of poker that has blinds, such as limit HE, or NL HE. Also suppose that I have decided to play my hand, and no one has entered the pot in front of me.

Why would I ever want to limp in?

It seems to me that by limping, as opposed to raising, I am just taking money out of my pocket and giving it to the big blind.

Why would I do this?

I am aware that S+M consistently recommend limping in with certain hands in certain situations. But what is the theoretical justification for it?