It’s a file sync platform. Say you have files you want to access from your desktop, you install next cloud and place the files there. They sync up with the next cloud server, presumably your NAS.

Now let’s say you want to access those files from another machine. It could be a laptop, an Android phone, your friends, whatever. You just need to install the client and login, and there your files are, ready to sync to the new device.

Great use case would be syncing your computers user folders, such as my documents, desktop, etc. If you have to wipe your computer and start over, at least those items are preserved and easy to restore.

Otherwise sharing files with other machines in general is the main use.