A simple bluffing hand

Turn raise serves several purposes. You are legitimately semibluffing with 12-18 outs, disguising your hand for when you do hit considering he likes his hand enough to call your raise, you have a good shot of taking the pot down right then and there, and you will likely get a free showdown if you hit your pair outs (which will likely not be good if you DO get called, but have some showdown value nonetheless). If you raise to pot or close on the turn, he is going to have to expect to call on the river if he continues, and if he has a lone pair of 8's it is doubtful he will want to do so. Your bet on the river is the full price of showdown, and he knows that is all he has to put in, while if he calls a bet on the turn he has no idea what the actual price of showdown will be if he decides to continue with his marginal holding.