Reddit has pathetic ad revenue per user versus every other social media platform. Part of this is likely due to anonymous users that they lack data on for proper targeting. This is their attempt to "fix" that.
If a store sells geeky/sci-fi clothing and gifts then I found it is way better to just target anyone that visits /r/startrek and some other relevant subreddits. No need to build profiles on users.
I found Facebook to be a lot harder to accurately target users as you are relying on Facebook's profile of a user. I got a lot more reach, but much less conversion.
Facebook has over a decade of data on its users, no session id will match that.
edit: Also knowing users emails will allows advertisers to match the reddit accounts to their own data stores on users.