Why do snooker players use chalk?

Answer 1

To help prevent the stick from slipping when it hits the ball.

*Added - The chalk increases friction and is very important to be applied to the cue tip for anything other that a perfect center stroke to the cue ball.

Answer 2

The cue tip is leather, and after a while it gets shiny through usage,

so chalk makes it rougher so the tip grips the ball better,

and making contact does not slide off and cause a mis-cue.