My impression from when I’ve encountered this is that it is an attempt to repel bots.
hmm bots don’t use keyboard or mouse copy & paste so I don’t see how that makes sense?
my impression is this is just stupid product managers who don’t understand why it’s a bad idea to force all your users to manually type out their passwords or email addresses just because of the 0.1% of people who would copy and paste one with an error in.
Sadly this doesn’t work reliably - an increasing number of sites still manage to block it. Also it prevents other sites from working properly.