i think the logic behind it is that by raising the turn and checking behind the river, you charge the small blind an extra BB to draw to the flush while costing yourself nothing since you're likely calling down either way. it is better than just calling down because you still have outs to improve if you're behind MP. so while generally the extra BB goes to MP, you'll occasionally suck out and win an extra BB that you wouldn't have won if you just called down while putting in the same # of bets.