Because most things like this are measured in average power per year and it is useful for comparison. Different technologies produce energy at different rates. Solar, only when the sun is up. How would you compare it to wind which has different rules?
Taken to an extreme, consider some hypothetical new technology that produced 50 Gigawatts of energy, but did it in a second and then took a year to recharge before doing it again. Would it be more useful to say it had a 50 Gigawatt capacity or that it provided 50 Gigawatts of power per year when trying to compare it to other technologies?
Edit: I hope nobody would use my hypothetical technology… Boom!
There is actually woman pages if you have Emacs installed (which used to be the norm). It stands for WithOut Man Pages: https://www.gnu.org/software/emacs/manual/html_mono/woman.html