Comparators have tile entities to store their power level. This is different than all other redstone components, for example redstone dust.
The only case where this might get used is if a chain of subtracting comparators with nothing in between was placed on a command block that outputs a signal strength of more than 15 (using redstone in command block contraptions is discouraged anyway and I've never seen this special arrangement being used) or when trying to read a power level over 15 from a trapped chest (which can only happen if at least 16 players open the same trapped chest at once, which should be an extremely rare occurrence).
Having a tile entity in comparators *might* also be needed to read the rotation of an item frame or the fill percentage of a minecart chest/hopper, but it might not be needed, this was discussed in the Eigencraft Discord, but not with no clear conclusion.

If this extremely unlikely case can be ignored and the third one does not need a tile entity, then performance can be improved by saving the power level in a block state instead.

(split off of MC-159902)