Why do 2+2ers raise the flop with middle pair?

There's a post on the first page right now where the IOP has 9d7d on the button in a unraised multiway pot with a flop Ac9c5h. CO bets, and one poster advised that the OP raise as a "standard" online move.

I didn't reply there because I want to make this question more general: in what situations should I consider raising middle pair on the flop? What am I looking for? I know that pot size, number of players, flop texture, etc. should be the factors that I consider--but what specifically is helpful? More players or less players? A drawish board or a lot of blanks?

And, in general, what is the objective? Am I looking for a free turn card? Should I do it when I have, say, an additional flush draw to add deception to my hand? Is it just because a lot of the time MHIG?

Or am I mistaken in thinking that this is a common move?

John