It enhances the experience on Twitter for those that desire more features: a thread feature (which is /really/ nice IMHO), ability to cancel tweets after send, more color themes and customizable icons, etc.
The Twitter "as we know it" will likely always remain free and ad-driven, but if you want an enhanced experience, you're going to have to pay for it. And that's fine by me.
The one issue is that chipping off existing features would cause major backlash. While removing low-quality replies and other spam is nice, it relies on even higher-quality moderation, to provide adequate incentive not to lose out on a purchase, but also to avoid regulatory issues, as this now becomes a product.
When it comes to adding features to sweeten the deal, however, there are very few that can really be added that provide a tangible benefit. The only feature I can think of is being able to edit tweets - or more accurately, embed a second tweet, similar to a quote-tweet, to provide an update. This would allow people to provide context, retract claims, or other uses, especially when a tweet gets attention or controversy, and could go a small way in reducing toxicity.