New Reddit is for ads. If you turn off adblock and compare old and new, you'll probably notice that there's more prime screen space for "recommended" content with media support.
There's still the sidebar carried over from old that can be used for basic media ads and the banner for sponsored posts.
In-feed sponsored posts are also a carry over, but the new feed has fully expanded thumbnails instead of mostly text which allows for autoplaying media ads (which is one of the most profitable ad formats). Old in-feed ads are primarily text.
New homepage has the trending thumbnails up top which works well with the same ad format as the sidebar.
New comments section truncates comments and has more fully expanded thumbnail recommended content below, which is another placement for in-feed ads.
Awards are much more prominently displayed everywhere, which is another monetization feature since they can be bought or earned from advertising.
The technical currently unrealized benefit with all the constant rerenders is the ability to bypass adblock rules by dynamically changing the underlying layout until the ad renders successfully.
Of course the web version being bad to funnel people into first party Reddit apps also helps their advertising angle because you can't easily adblock in app and 3rd party Reddit apps don't render ads.