Besides your instrument. Any musician can do that. What makes you irreplaceable in your band?
This is important.
Music is a highly competitive industry, and it's no secret band lineups change all the time. Unless you happen to be a prophet of the guitar like Hendrix, being "good" isn't enough to ensure you stay in a band. Youtube practicly has a dedicated channel called "12 Year Olds Who Are Better At Your Instrument Than You Will Ever Be". A Total musician, the kind of person you always want in your band, contributes something that can't be replaced by someone who can read guitar tabs.
What would qualify as something making you more valuable to the band?
-Writing the entire story behind a 4-album sequence. Would Coheed & Cambria have existed if the story writer left?
-Being a master promoter. Do you run the merch booth like a boss and have a knack for getting people excited about your music? That's magic, baby.
-Managing inventory and planning all the shows for the band.
-Doing all the graphic design and website building for the band.
-Recording good to great demos all yourself at the band practice space.
-Knowing boatloads more about business/money than the rest of the band.
-Repairing all the band's instruments.
-Illustrating all the band's merch, posters, and albums.
All of these things are more difficult to replace than simply "doing your job" on your instrument. Skills like these add value to both the band and yourself. The more business your band can do in-house, the less time and money you need to spend on outside help, allowing you to spend money on the really important things.
The best way to ensure your stability is to be invaluable.