Like Matt said this is just #churning for sign up bonuses/miles. 
Show me an incentive and I’ll show you an outcome. 
Most sign up for a year, meet the minimum spend, get bonus, then downgrade/cancel when the Annual Fee comes up the following year. 
They do not carry a balance if they know what they’re doing, and there’s a lot of wasteful “manufactured spending” (think buying rolls of quarters with a card then depositing them back into a bank, gift cards, etc)

I haven’t paid for a flight with money since 2015 when I learned about it. Not for everyone though..