This isn't something that's particularly obvious unless its pointed out, poly tunnels work on two fronts:

1) extend the growing season/climate control

2) scatter direct light into bright ambient light meaning that plants aren't scorched.

"leafy" plants like pumpkins, tomatoes, cucumbers etc "wilt" when in direct sun. they go floppy to stop evaporation. THis means they don't grow as fast. If you even out that light so its not directly heating the living shit out of the leaves, they grow faster, because they have less time in stress.