Seizing the iniative with a hand has value against bad players.

Raising a hand preflop is a kind of semi bluff that has folding equity inherently.

All poker hands have 'favourite environments' where their equity is strongest; A9o's happens to be heads up.

And in the case of A9o, in the right situation, calling is probably correct, but raising is probably better (eg; you are playing in an incredibly loose game where 9/10 players call with essentially random hands; A9 is definately playable otb since it has equity in this situation; but then you should be raising it for value anyway).