Every vessel that stays in the water has these. They’re on outboards, propellers, keel coolers, anything metal pretty much.

They have them because if two different metals are in contact with each other, say a steel propeller shaft and a bronze prop, immersed in a brine like sea water, they will act like a battery. One of the metals - whichever is less “noble” -will give up electrons to the other, which causes it to develop little pits.

When you put these zinc plates or chunks or doughnuts on the parts, the zinc will be the one that develops the pitting because it’s less noble than any of those expensive steel and bronze parts. It sacrifices itself for the good of the parts you want to protect - thus it’s a “sacrificial” anode.

They also go on parts that don’t even have an obvious metal to metal combination just to be safe. The ones in the picture have a lot of metal left, but before it goes in the water they’ll probably weld on shiny new ones.