Essentially it boils down to - ways in which they can turn users into money.

• Control the content you see - Especially ads which provide income. Also injecting posts into your stream regardless of preferences to direct views towards sponsors preferences or to try to extend engagement.
• More visibility over user activity - It gives them better tools to manipulate users habits (that pesky engagement), and better (for them), user telemetry can be sold (people who like X like Y is useful knowledge but it can go far deeper than this).
• They want to discourage content that discourages advertisers - mostly this is NSFW content (since the advertisers don't want their ads showing beside NSFW content) but I expect it'll begin to span more than that over time.

With the elimination of third party apps, you can bet that 'old Reddit' is on the chopping-block soon too (mostly to boost ad views)