There are various reasons, one of them is like mentioned above that I like to re-utilize old disks after upgrading them, I find this more cost effective than just upgrading the existing server to even larger disks, also and although I work in IT it's kind of a hobby for me, and finally my small way of supporting LT.
As for what they are used, there are two main servers that are always on, with VMs and the data I need day to day, they also serve as partial backups of each other, then I have mostly cold archive servers, 5 in total and for each of those there's also another one that acts as a full backup, another server for backup of the array for one the the main servers, which doesn't need daily backups, currently also a couple of Chia farms and a couple of test servers which for now share the same key since only use one at a time.