You can rent them. They have a bar across them like a Club locks a steering wheel.
It's a German thing.
I guess you can't tell, but the beach is right there. It's not a field
Strandkörbe.
You just sit in them. I only ever see old people use them. Or families with little kids, sometimes. Maybe it's good for collecting all your things.
Their sides also provide an astonishing amount of protection from wind so you can sit in them gar earlier to sunbath than you might want to lie on an open beach because the air temperature and wind might make it too cold!
But other than that yeah having your stuff in one of them/some of their side pockets is nice too.
Also they have a small parasol thingy at the top (don't ask me what the correct English name would be right now) which can share just your head/eyes! (Also obviously to allow slightly longer sunbathing/sitting at the beach without overheating)