No prizes for the clever answer “work on a roof”. They do indeed do that but I’m betting that you have very little idea as to what they do actually do when they are. The roofer like a Gloucester Roofer available at http://gloucester-roofer.co.uk/ do more than just bang nails into a slate tile. They can immediately see with their trained eyes exactly what is wrong and get the job done to fix it.
For the most part it will be a bit of touch up job and quick fix of any holes to damage that needs doing. It might even involve a bit of bird shooing if the pesky little things (or sometimes big if it’s a blackback gull) have managed to get themselves ensconced in the eves.
If there is too much damage then it becomes a much bigger job and the roof will need to be removed and replaced. All the shingles will need to come off and a whole new set put back on. This is a big job but a real pro will soon have it done for you. Expect Ladders, harnesses and even a cherry picker if it’s a three storey house. They will even have a go at cleaning out a gutter as well.