This is what people fail to appreciate with the Pogba transfer. It was better for Utd to pay £89M and break the world record than not do so.
The fee generates publicity. It puts the club on the front and back pages. Even though Utd aren't presently competing for the top honours, it makes a statement of intent and keeps the profile of the club high. It says "we are still the biggest club."
This is attractive to fans and to sponsors. It helps sell shirts; it helps ensure money keeps flowing into the club. Sponsors want maximum publicity; they also want to be associated with the show of wealth and ambition - this helps sell their products. They'd rather be synonymous with the richest club in the world than say, the most successful in England last season, Leicester City. We live in times where people want to see an ostentatious show of wealth - it's a career move for some (look at the likes of the Karshardians etc). You don't have to show that you're successful, just wealthy. People aren't interested whether it's deserved or not - just that you've got it. Football is a bling sport nowadays.
If buying Pogba draws in an extra £50M in income each year for the next two or three years (which it will, if you believe the accountants), then it's good business. And football is primarily about business these days.