Why do referees/umpires wear numbers?

Just what the title says. Why do referees/umpires/other on-field officials in baseball, football, basketball and hockey have numbers on their uniforms, like the players do? AFAICT this seems to be limited to professional leagues in North America. (I don’t believe I’ve ever seen them on officials for college or international games.)

For the same reason as players do, for identification. Professional sports officials are evaluated on their job performance and the numbers are a shorthand for identifying them.

[checks first to see that we’re not in GQ] So that when the audience members inevitably turn on them, they’ll be able to be specific.

“Offensive foul?! Number 32, you SUCK!”