The "Other" category consists of necessary stuff like the OS and caches, but it also has a lot of leftover junk that you can't purge without doing the reset and restore. Some of this is stuff like botched app and song downloads.
In my experience, the "Other" category seems to grow larger if you use iTunes Match, because the songs you play using Match are downloaded to the phone and I believe that the music library data for Match (which can be hundreds of MBs) is stored as "Other" data. If you accumulate a large number of songs, you can purge them to free up storage space. But, for any songs that did not download completely or were interrupted, those get orphaned and there's no easy way of manually deleting them.
I've just settled on a routine of doing a factory reset and restore from backup every other month. It takes about that long for the "Other" category to balloon out to over 2.5 GB, and I have some other badly written apps that also hog up space that they won't release without a reset and restore.
You can also free up storage space by deleting messages (with their attachments), deleting your music files especially if you use iTunes Match, and yes deleting apps.