Choice and competition is the only thing they offer. There appears no real USP with Monarch, they offer the same routes from the same airports. There also seems to be a hangover from the charter days which places the airline in the locations it currently operates from, and again offering the same routes.
Brits just need/want options, reality is we don't 'need' anything more than is currently available at the larger airports all within two hours of most of the UK population.
If an airline can offer a little more convienience and competitive cost then the seats are there for the taking.
If Monarch had chosen DSA over LBA a few years ago the story might be different...at least in the Yorkshire region. Flybe have since taken the popular routes. Would this have been enough to change Monarch's fortunes? Maybe not, people will pay a small premium for convienience but a significant difference in price would see people travelling an hour North or South for the cheaper fares - they've probably secured the market rate bookings at LBA.
Me personally, not used Monarch for 6 years, they are always more expensive than FR/LS and at the end of the day it's just a ride from A-B.