Because we like to maximize those miles, above all else.
Some time ago, actually a really long time ago, we wrote an article about when it's a good time to redeem miles. But, honestly, I'm lazy, you're lazy and no one's got time to look for formulas. This calculator should help you figure out whether booking those award miles or spending cash is a better proposition.
Even better, it accounts for the miles that wouldn't otherwise be earned if you decide to book an award ticket.
Most of these calculations rest on how much one mile is worth. Though there is some variance on this, the assumed value here is 1.4 cents. It is based off the fact that 25,000 miles, or what's needed for a standard round-trip ticket, is worth $350. (In jargon, this is often referred to as cost per mile, or CPM.)
Please keep in mind it doesn't take into account anything like atypical redemption schedules (i.e. JetBlue) and checked bag fees. Because that's now a thing.