1) breaking news often appears on Twitter before it hits any of the wires or other news sources. I picked up a Tweet from someone in Denmark last week about s story, via Twitter I found a Danish speaker who could translate a source document and who could do an interview and we had a story written while it was still breaking on other news sources
2) I don't follow many celebrities, most of the people I follow have expertise in particular areas. That means if they recommend an article, it's normally worth reading - that saves a hell of a lot of effort in research, where I might have to trawl half a dozen or so before I find something worthwhile
3) If I want someone with particular knowledge (eg a Danish speaker), it's a lot quicker to find someone on Twitter than ringing round lots of people
4) By following Twitter trends, I can see what's interesting. It helps with tagging stories and what to use for search engine purposes
5) It's a great way to promote stories that I've written - particularly when they're picked up and retweeted.
6) It's great for picking up info out of hours - you're no longer dependent on 9 to 5 contacts but can find someone at all times of the night
It's not essential, but it makes my job a whole lot easier