Here's a few reasons ... depending on the time of the season any of them could be the "main reason."
1. It's preseason, so the coaching staff doesn't want to take any chances with key players and veterans. Gase even said that vets of 8 or more years are likely to be rested periodically. Players recovering, players with minor issues, they hold them out unless they absolutely need the reps.
2. There's a lot of people to evaluate, letting some of those you already know about take a day off isn't a bad thing, you then get to look at guys who need more reps.
3. Our front office thinks a lot of it's own sports science and health programs, so it tends to get injured players on the cheap and hope to rehabilitate them for great value. Sometimes it works, sometimes it doesn't, but it always creates a roster with more injured players.
4. Players aren't yet in real football shape, so their bodies get tweaked and it's good to give them some rest.
5. It's darn hot and humid in South Florida this time of year ... muscles have problems, best not to push it too fast just yet.
6. Bees man ... it's the danged bees.