Great question.
In essence, it is a one off hit of £30m. I'll resist getting technical. The simple answer is there can be a lot of reasons, including looking ahead to FFP and EUFA qualification in 2 or 3 years. Bear in mind that the season just finished could show a profit of £80m, even more, depending on what happens with the earlier transfer window.
That new profit puts a higher price on the club next year. It also gives new owners more scope so yes, it does help a sale.
In terms of the "any other reason", yes it can help lower Ashley's tax bill. Forget the £91m loss, the loss "after tax" as it is described of £40m reduces his tax in this financial year by 40% of £41m, put another way, he has saved £16m.