In order to create a file on a filesystem, you need some free space and a free inode (which keeps track of, among other things, where the file actually resides on the disk). You cannot create files if there is no space left or there are no free inodes. Inodes are created when the filesystem is initialized. They are part of the filesystem overhead. It is not possible to add inodes to an already existing filesystem.
When creating new filesystems, we usually reduce the number of inodes in order to get a few percent more usable space. Even then, we're typically at only about 30% inode usage when space usage approaches 100%.
But some users have a lot of very small files. At this rate, you can run out of inodes before running out of disk space. It would be quote frustrating to look at a filesystem with 10's or 100's of MB of space available and not be able to create new files there.
Usually, such users know who they are and have run into such problems already. If you expect to have high inode usage, you can put "increase inode count" down for special considerations.