They bring in a load of tourists into Manchester which is good for the economy. I'm sure they employ a good few locals as well which helps people out.
They look really festive and have become a centre-piece for the city centre at Christmas and I'm sure there's millions of photos beaming the centre of town around the world which will also boost it's image and encourage people to visit.
Yes, the food, drinks etc are expensive but the good news is that you don't have to go. You can choose not to go, go once or go every week - the choice is yours!
They aren't my cup of tea, I feel I've done them enough over the years, but they are a HUGE draw for people. I never hear about people going to the Liverpool Christmas Markets, Leeds Christmas Markets, Newcastle Christmas Markets, Birmingham Christmas Markets, London Christmas Markets etc!