Brake fluid is hydroscopic i.e. it tries to absorb water. Mostly this happens through the flexible rubber hoses down at the wheels. This then does 2 things; one it promotes corrosion in your calipers at the wheels that stop the car .. so they sieze and stay stuck on. Brakes pads and discs then wear out quicker, the handling goes to pot at the limit of handling because one wheel is holding on the brakes, your ABS and stability systems stop working correctly, you need to buy new calipers (or more likely the next owner). The second thing is that it lowers the boiling point of the brake fluid so if you are doing hard braking (and that means trying to go fast down long steep hills that take 5 minutes to go down.. so pretty rare to most folk in UK but something I have done in Scotland and Wales) then the brake fluid boils and you lose your brakes as steam does not acts as a good fluid..
Money making by dealers... nope. However for 90% of owners it is .. well in the same way that changing engine oil is money making? i.e. it benefits owners in 7-12th year of ownership .. not the first 3-4 years where the dealer services the car.
I try to change every 2 years.. hey even every year but I am an ex brake test/design engineer. Good mechanics take a cup full from each wheel when changing brake pads as a cheap way to get rid of the "bad wet" fluid. Mind you as 95% of mechanics do not lubricate under the piston seals on the calipers then they sieze due to water creeping in from that way so the wet brake fluid is not what fails them. So again.. 95% of folk will not benefit. So I guess that is a waste of money as you need to maintain the whole system and the manufacturer's instructions do not do that.. they only want it to last 3-4 years.