There are many reasons why one would do a mileage run, the most obvious one being that you're that one well-planned trip short of renewing your status, but sometimes you do it because you have a status challenge, or it's a shortcut to elite status after switching FFPs/alliances etc. One might also have a gap year in their PfL journey (for instance, due to the birth of a child) that is easily made up over a couple of weekends. Finally, a mileage run can be had for the sheer fun of it - I, for instance, have never flown Saudia and wouldn't mind doing the TUN run just for the fun of trying out a new product and reducing the pressure on renewing elite status next year.

If we're getting philosophical about the topic, I will cite two of my favourite quotes from French authors:

"Neither the sun nor death can be looked at with a steady eye." - La Rochefoucauld

"Death is easier to bear without thinking of it, than the thought of death without peril." - Blaise Pascal

While it's good to be reminded about the transient nature of our pleasures every now and then, I find it profoundly morose, even pathological, to evaluate all of life's activities through the lens of a potential imminent death. And frankly, if I were to find out that I will die tomorrow, the 1000+ hours of flying I've done in the past decade (including a handful of mileage runs) would be among activities I regret the least.

A signature-worthy thought if there ever was one.