My host takes weekly backups but I also take regular backups as you can't always rely on your host. I take my own backups because it's another layer of security just in case my host has problems with their generated backups. A lot of money and time will be wasted if backups are not made regularly and it's important to make backups on multiple servers in case one server fails.
I am doing weekly backups on my site for now , its pretty new so I won't lose much if it goes down somehow. When I had a forum running that had over 10k posts I backed up my site daily to not lose even one single post.
Prophpbb does take backups but I'm not sure how often. I do know for sure that if you want a back up you have to email the host owner for permission unless you're on their paid plan. I think that's the only way you can take backups whenever you want is if you're on their paid plan. Otherwise you have to email the owner for permission.