Explanation
It is the honest denominator for anything describing how many people a campaign touched, and it is the one most likely to be quietly replaced by impressions in a report, because impressions is the larger number.
On short-form, reach growth and follower growth are only loosely related. These platforms distribute heavily to non-followers, so an account can reach a very large number of people while converting almost none of them, which is a profile problem rather than a content one.
The common mistake
Using reach as a proxy for audience. Somebody who scrolled past a video counts in reach and has no relationship with the account, which is why follower conversion has to be measured separately.