It doesn't "help".

There is no functional advantage. Thomson offers two types (setback and straight) simply to allow for better seat positioning.

I'm sure Thomson would argue their setback post with the bend in the post (rather than the standard offset seatclamp) is stronger and less likely to fail (and who's really to argue with them because they make an outstanding product), but their straight posts aren't about to fail either.

I guess a setback post gets your weight further back over the rear wheel, but if that also means putting you in a bad position for pedaling (position relative to pedals) then that's worthless.