Mostly just the males get large teeth during the spawning run. The purpose for this is to fight off others who are competing for nesting sites and/or territory.

When they are holding in pools in larger streams, the males will chase and bite eachother which I guess would be a territorial thing. A couple weeks ago on a small stream, I was obseving a somewhat fresh looking male(with teeth of course) and female chum sitting over a redd, and got to see how the teeth are really used. Behind the nest, there were proably 5 other chums (mostly male and moldy looking) sitting in the pool below. Every so often, one of them would swim up to the nest and try to nudge the other male away. This provoked a fight, in which the brighter dominant fish would bite the intruder and chase it back to the end of the pool. While I was wathing this I actually saw the domint fish bite one of the older ones so much it started floating belly up.

Usually If you look at a spawning chum you will notice they will have bite marks and battle scars around thir head. The only other functions for large teeth are to fray fishing line and to scare the guy handling them.