How does b3 prevent e5 here? [FEN "rnbq1rk1/ppp1b1pp/3ppn2/5p2/2PP4/5NP1/PP2PPBP/RNBQ1RK1 w - - 0 1"]


White plan is to play b3 and then Ba3 to render the move e5. But how does it prevents e5?
Currently e5 is not playable due to pawn loss. b3 is good as if ever e5 occurs after preparing for it with Nbd7, dxe5 dxe5 would make the long diagonal accessible to the bishop forever. In double fianchetto positions, an open centre is desired for maximum range of bishops.

Obviously, It's not strict, e5 is a very important break in many dutch positions. 

This is an opening position. Many things are possible here. You can read some books on classical dutch for more insight into this position.