Seeing @DobieLover’s post reminded me of another reason: so you can hang out with your chickens.
I too have a chair in one of my full height runs and a tree stump in the other. It is very enjoyable to hang out in there with them.
Access.
Access for maintenance, to retrieve eggs, to retrieve a dead or sick chicken.
I have two runs that are only two foot high and I made them with hinged lids so I still have access.