It’s to prevent you screenshotting snaps / disappearing messages sent to you or to prevent screenshotting licensed content. This is good for 99% of the people.
No. "Disappearing" messages are a farce. Disabling other people's screenshot capability so someone can be made to believe that their message will disappear is absurd. If you want a message that disappears, tell them in person and pat them down for a wire. Also there's nothing wrong with simply screenshotting licensed content. I'm allowed to do so for my own personal use. If someone is distributing licensed content that they are not licensed to do so, that's a matter for the courts.
In general paying content producers allows them to produce more content. If it's neutral for the (legitimate) end user, but positive for the creator, why would you not use it?
Because they want to access the content, which might not be available otherwise.
Netflix seems to be quite popular, and it's users don't seem to be bothered by the DRM - try taking a screenshot of it on the desktop, you'll get a black picture.
Like how they stopped showing movies on TV once VCRs entered the market, or stopped playing music on the radio once home taping killed it. If only we had had DRM on radios and TVs back then, perhaps there would still be some entertainment industry left.