A means for rapidly selecting and loading an advert which will then be blocked by the users' browser.
(Although even without adblock, are there actually all that many overt ads on Reddit, other than the sidebar ad which nearly always is an ad for a different subreddit?)
Unfortunately, they're probably using the app because they got annoyed by how much the app is forced onto you while using the web version.
I however, will not relent.
I still use old.reddit.com, even on mobile, because the new React version is slow and constantly tries to get me to download the app so that they can show me ads.
(Although even without adblock, are there actually all that many overt ads on Reddit, other than the sidebar ad which nearly always is an ad for a different subreddit?)