One advantage for horses to living as domestic animals is that they have consistent resources provided to them and peace to enjoy them.
Most horses adjust to most conditions, but some are sure nice, one is having their own safe, comfortable individual little corner of the world and it’s resources all to themselves, at least for part of the time.
The more options we have to manage our horses, the happier we can manage them.
Stalls and individual pens can be part of that.