I think in addition it's about shareholder value - I would argue that telling your shareholders and investors "We have 1000 paying subscribers" would be considered more valuable than "We have made 1000 sales", even if the total value of either is equal.

Subscribers have a lot of "lag", as the parent says, with a lot of people continuing to pay for a subscription they don't use anymore, or where they have to sit out a subscription for a full year.