[adouglas]

The short version:

Jets determine how much fuel goes into your carb. Bigger jets, more fuel.

But fuel and air need to be kept in the correct ratio. Dump more fuel in without doing anything else and you wind up with a too-rich mixture.

Flip side of the same coin: when you change airflow by installing a freer-flowing exhaust and/or intake, you add air. If you don't do anything else, that makes your mixture too lean.

So you install bigger jets along with your intake/exhaust mods to get the ratio back where it's supposed to be.

More air + more fuel = more power. That's why we do it.