Glossary is a good thing if you are doing a LONG book. I use it so I can keep track of systems, characters, etc. So if I bring back a character that was there 30-40 chapters ago, I can simply go look up his/her name, etc.
For me, its best usage is to keep track of characters when doing a book I plan to pass 200 chapters with. I would forget less important side characters with my scatterbrain.
RE to @Corty and greyblob:"Glossary is a good thing if you are doing a LONG book" I have 108 k words and it is half away arc 1. and I am planning arc 2. Is it long or short book you reckon?
Do it if you are in for the long haul. That is a length where you can already have some instances when you have to search previous chapters to fact-check yourself.
I started doing glossaries because it was a much faster way to check my previous points in the story and not contradict myself.