[Schneids]

What a lot of people have said are the essential reasons why. Also to add: Say the raiser bets, and the SB calls and you call. What do you like doing on the turn if a relatively safe card comes? Checking? Leading into the raiser? Firstly, checking sucks because a lot of the time the PFR may very well check behind if he has no pair, and likewise, he may bet with no pair, so now you're really in a quandary when the action gets back to you. Leading the turn sucks if the PFR is at all good because your pf/flop/turn line looks exactly like this type of hand (unless you start playing flopped top pair this way too) so he should be able to raise you with a LOT of hands, both either as a bluff or for value.

So in part through process of elimination, if the hand is going to be continued, there's a lot of merit to checkraising the flop (not to mention meta game benefits of having this flop checkraise mean more than just top pair or flush draw or that type of thing). If the SB is the only turn caller you obviously check the river and give up (and make note if he did flop a pair and only check called) since it's likely he either has no pair and is folding, or has passively played a pair the whole way.