It doesn't. Watts were a mistake by whatever committee it was that standardized unit names. Power should not have been given a unit; it should have been left as ∆energy/time just as velocity is distance/time.

Joule is a derived unit, it is kg*m^2/s^2. There are lots of derived units, like hertz and newton, because they useful than writing out the whole thing. Electronics would be really annoying if had to write out volt, ohm, and watts (ampere is base unit, coulomb is derived).

It's shorthand for a Joule (unit of energy) per second (unit of time). Watt is the problem with that?