It not only lets them stretch their legs, get a bite or two of green
grass, etc. It lets them cough out any "road trash" that's been
accumulating in their bronchial system. It's hard for horses to do
this in a trailer.
Some horses are reluctant to urinate in a trailer and will hold it
longer than is good for them unless they are let off and can make a
"nature call" on grass.
Besides, it makes the trip more pleasant for them. Don't you like to
get out of the truck, walk a bit, and visit the rest room from time
to time?
The best places are public rest areas (away from the main traffic
flow) or grassy fields next to service stations. Very few of them
mind you doing this as long as you clean up any "road apples."
Of course, offer your horses clean water at every opportunity, and
plenty of good hay. Hosing down is not necessary, although it might
help on a particularly hot trip.
Good luck!
--Joe Long jlong@hiwaay.net