Judie's answer covers most of the main points. It's worth pointing out, though, the reason why plasterers (and others) like insurance work:
If you ask a tradesman for a quote on a job, he might say "I can do it for a grand". When you look doubtful and say that you'll get back to him, he'll say "Tell you what, guv, I'll be cutting my own throat but I reckon I could do it for �800". So 'private' work only earns him a fairly low amount. If, however, before getting a quote, you tell the tradesman that the insurance company will be paying, he'll probably say, "Lucky you were insured mate. That's an expensive job you've got there. I reckon I'll have to bill them for at least �1500". So, 'insurance' work work can bring in nearly double the earnings of 'private' work. (The insurance companies are, of course, wise to this - which is why they normally require a minimum of three quotes - but, all the same, tradesmen usually get paid much more for 'insurance' jobs than for others).
Anyone who doubts the validity of what I've just written should try taking a car, with a large dent in the wing, into any garage. I guarantee that, when you ask for a quote, you won't immediately be given a figure. Instead you'll receive a question: "Is it an insurance job?"
Chris