My favorite thing about retention is the leverage it gives you. My favorite analogy is the "leaky bucket": You have bucket with holes in it. It's not holding water. You can pour water in as fast as you can and you might be able to get growth... But as soon as you stop, the bucket begins to leak out.

Or you can plug holes. Make users happier. Work on re-activation. Give value.

If you work on top-of-the-funnel stuff (more water) FIRST and then try to solve retention second, there's a lot of waste. You've leaked users. You've created a group of folks who don't talk enthusiastically about your product.

If you work on retention first, all subsequent marketing efforts are more powerful. Word of mouth is more powerful.