Why retention is important:
If you have 7% revenue churn in a month (you lose 7% of paying users), it doesn't seem like a lot (you still keep 93% of your revenue), but that adds up to 58% in a year. So if you're a $1m/year startup, each year you have to figure out how to make up for $580k in lost revenue, and grow on top of that.
The more you grow, the harder and more expensive it will be for marketing to gain the amount of users you have lost.
Don't keep pouring water into a leaky bucket.