What is the advantage of a 90min. boil vs. a 60 min. boil?
When a majority of the base malt is pils, it’s common practice for a longer boil to drive off DMS.
Other reasons would be to get down to your correct post boil volume or to caramelize your wort more for a darker maltier beer like a Belgian Dubbel for example.
A longer boil can allow maximized efficiency of hop bittering. I’ve read that hops being exposure to proteins before hot break is complete can impact bittering potentials, which may be why FWHs don’t provide the same sensation of bitterness that 60 minute hops do.
But that’s not something most people would (or should) worry about.
Another big reason for a long boil is it allows you to concentrate your wort to get high sugar levels while still maintaining high efficiency of extraction. Mash efficiency drops with higher gravity, and this is one traditional way to counteract that.