The reason this is so--
1.)plants aren't sending so much nutrients out to their leaves, its all going into the root system. ( less shock will be involved)
2.) with cooler temps the ground stays moist longer , so less watering is involved
3.) gets winter moisture and spring rains to start the root system off rightsysteoff right.
4.) they go dormant
You can also plant grass in the fall and have great results, because of many reasons I listed above. Its a good time to roll out your lawn, but if you do this make sure it is aerated to break up the compaction you just caused. Bare spots in the yard can be seeded at this time too (overseeding).