My current employer does bonuses based on both individual and company performance, and I think they can be sizeable, but I pointedly never did the math when accepting the offer or since then. I don't want to know.

Base salary is in the contract, bonuses aren't. Base salary gives me a floor for how competitive the offer is. Bonuses, if they actually materialize, could theoretically help keep me from wandering off to a higher-paying job -- but I'm probably not going to do that math until I want to leave anyhow.

It's basically like stock options and whatnot. Lottery tickets.