I really can't imagine any airline leaving without giving their full load to check-in on time. However, if everyone's there, why not get going?
Ground controllers will let them have an earlier slot, if they have one, so as to keep some free space for later, if it's needed.
They can arrive earlier, giving themselves more turnaround time at the other end. That, of course, assumes they can get an early landing slot.
Most importantly, more time in the air means the pilot can use a lighter right foot and save a fair bit of fuel. This benefits us all!
btw Checking in close to the last minute dramatically reduces shopping time airside.
This may, or may not, be a good thing.