Churn rate is what gets lost between sales and growth. If you acquire 100 customers in a month and lose 5 of them, monthly customer churn is 5%. The same percentage on revenue (revenue churn) tells you whether the customers leaving are bringing more or less than average money with them.
For agencies, churn is the metric that quietly decides whether a paid-media plan is sustainable. A 5% monthly churn rate means a fifth of customers are gone within a year — no acquisition spend survives that for long. Reducing churn by a single point is almost always cheaper than increasing acquisition by an equivalent amount.
Watch the two flavours: gross churn (lost revenue, no offsets) and net churn (lost revenue minus expansion from existing customers). A SaaS with negative net churn is growing without acquisition; one with high gross and negative net is still in trouble.