They are a way for airlines to create value out of thin air with their own fiat currency. For the average consumer the miles create less of a pure economic efficient benefit and more of a psychic benefit - funny money bucket that accumulates to defray some trip expenses. Economically they'd be best off with an outright 2% back card.
For the minority of customers who are flexible on dates/locations and willing to plan in advance - most airline/hotel programs have sweet spots at the more premium level.
I use them to fly overseas business class refundable fares at discount. It's not free, but the taxes paid + foregone $ I could have gotten with a cash-back card ends up being 2-3x economy non-refundable fare instead of the 5-7x listed cash price if I bought the business class ticket outright.
It's generally a time-vs-money thing though in that to maximize airline/hotel programs you need to pay attention to various limited time offers for signup bonuses, spending bonuses, conversion bonuses, redemption bonuses, etc. Without those it's a very uphill battle.