[wrto4556]

Because only a better hand is going to call you. If you have the book, open it up.

Our chances of having the best hand is pretty decent (bet), its likely that a free card sill hurt us (bet), the pot is semilarge (bet), we have few ways of improving (check). Betting would be OK. But Sklansky gives this same example and says to be more inclined to check a pair of eights on this board. I'm assuming you're in early position and don't have much information...but betting A7 up front is correct (see example).

What about if 2 people were to act behind you? Is that the case where you check and bet/raise a good card on the turn if noone bet the flop...?