Why does my light switch have one hot wire in and two feeds to the same light?

I’m swapping out my light switch. It has three black wires and I originally thought it was one hot, one to the light and one to the other switches (where-ever they may be). This is how my 1st floor switches works.

However on further testing and much frustration I found the following (at least 2 switches and on 2nd floor)…

• one hot black wire into the top of the switch.
• two black wires on the bottom of the switch BOTH feeding to the same light fixture.
• I’ve replaced the light fixture before and don’t recall two black wires.

Any thoughts as to why there are two black wires from the switch feeding the light fixture?

Also this light fixture is part of a bunch that are controlled by two 15 amp breakers (e.g. I have to shut down both breakers to cut the power to this circuit). Can anyone explain why it was done this way?

Thanks
on-ca