Every airline offers this type of buy miles get some miles free offer. In some rare occasions they make sense. Here's one of them,
I'm lifetime United 1K ( yeah that is possible - it's a long story), and United just sent me an offer to buy miles and get up to 70% free (also 35% and 50% for lower levels), so I could purchase 30k miles and get 21K miles as a bonus for $1050.
Because United does not base rewards based flights on acutal fare, that purchase might make sense on United. Economy RT's (to anywhere in the lower 48) cost 25k miles.
So, a RT Dallas/Chicago departing tomorrow points cost is 25k points - $ cost is about $800. So one could buy 51,000 points and get $1600 in flight value for $1050. Becasue I'm 1k, my only cost for a RT beginning tomorrow would be something like $11,50 for the TSA fee. Someone with no FF status would pay $160 in fees.
It might not make sense for most advance purchase fares, but it sure would for unexpected no notice departures
