It's to confirm the remote employee is behind the computer. It's not always necessary that they be doing work if there's no work at that particular moment for them to do, but in that case they're still being paid for their time and for some reason these companies don't want their employees doing something else, like going to a baseball game. It's basically no different from an onsite working being at their desk even if they have nothing to do.

It's also the logical corporate response to Younger Millenial remote workers bragging about "working" from paradise instead of actually working. As to why management wants it...this is literally something that management has wanted dating back to the rise of civilization. They have power and want to exercise it even if it's not necessary or efficient.

The logical labor response should be to not work for companies that require remote employees to submit to constant visual supervision.

I'm not sure employees have the power to not really work at those kinds of places. Logically it should work out like that, but I don't think it does