Boiling for 90 minutes is recommended for all-grain brewers using pilsner malt. The pilsner malt contains higher levels of SMS, which get converted into DMS lending a cabbage flavor to the beer. SMS is volatile so the extended boil vaporizes off the SMS to reduce off flavors. It is not necessary for extract brewers (since it's already been vaporized) or all-grain brewers not using pilsner malts in their grain bill.