Metrics are like an onion: full of layers.
A simple question to ask can often be complicated to answer. Or we may find that we need to ask a different question. While it'd be lovely to have one simple metric to understand our business the reality is usually a lot more nuanced and interesting than it first seems.
Averages will deceive. Vanity metrics may wow — by, say, quoting a total user count and conveniently ignoring whether those users are actually paid, active, or leaving. Growth overall can hide declines in other areas or product lines and vice versa. The people on monthly subscriptions may be doing great, while revenue from add-ons may be declining. New users may be signing up in droves while existing users are churning. Mobile traffic may be different from desktop, online sales different from store sales... In practice, it's the hard questions we're usually asking even if we don't know how to ask them.
It pays to carefully consider what you're trying to learn and to understand all aspects of a business before putting faith in the numbers. And it may make sense to start with simpler metrics first.
Spend time with your data.