It’s rare among mammals in general even if it’s universal for sloths.
Most people think of evolution as a string of improvements, however you can get negative traits in a population from a large enough genetic bottleneck or spread by being next to a very useful mutation. So it’s not necessarily an advantage for them, it could have just been bad luck.
Alternatively, there could have been some selective pressure for it as a means of improving night vision or something, we just don’t know.
Most people think of evolution as a string of improvements, however you can get negative traits in a population from a large enough genetic bottleneck or spread by being next to a very useful mutation. So it’s not necessarily an advantage for them, it could have just been bad luck.
Alternatively, there could have been some selective pressure for it as a means of improving night vision or something, we just don’t know.