CAT5 is definitely overkill, but widely-available. I am sure that's the primary reason. Unless there's a very unusual need, all telephone/data wiring is 24 gauge. The exception is CAT6, which is 23 gauge.
22 gauge was the norm for surface wiring that was run around baseboards to make it more resilient to damage during installation.
Regardless, it's more than enough to handle the small control circuit loads as Paul mentioned.
"But the guy at Home Depot said it would work."