Also it is a good idea. Now with that said and a little bit of time on your part you can do it yourself a lot cheaper.
Basically all they are doing is opening each bleeder and letting the old fluid flow out while adding new fluid. When the fluid comes out clear or what ever color they are adding they are done.
You can do this with a $30 to $50 tool and about half an hour. If they are charging over $90 then I would do it myself.
There are several Youtube videos on how to do it.
The reason is the fluid collects moisture over time which brakes down the brake fluid. At least that is what I think. I do this to all my cars, if I keep them over 6 years.