I assume they hope to attract more advertising money this way.
There was a few months where it was required, but it isn't anymore.
I’ve been in tests where the entire site is gated, demanding I download the app. I’ve been in tests where SFW content is marked as NSFW, demanding I log in. Etc.
I still routinely run into it on my phone where I cannot view the site at all unless I download the reddit app, especially if its a NSFW subreddit
Everything else is secondary and most product managers don't view a/b tests as anything other than experiments on ways to increase those stats.