It is good practice And meets building regulations to flush a system when installing a new boiler in exchange for an old one. If not done the instalation does not comply with Building regs and the maufacture will not honour the guarantee if the benchmark procedures have not been adheared to.
It is also often needed in existing systems when if they have not been regulary maintained and inhibitor levels checked and topped up annually and a build up of scale and sludge has occoured often noticed due to cold spots on radiators, rads not getting hot enough, noises from the boiler, having to bleed rads too often and premature pump failures. I Charge �40 per rad here in Yorkshire. You can flush the system you're self as Legend002 suggests but he does'nt mention a powerflush machine costs around �1000 to buy plus chemical cleaning agents and inhibitors and then you need to know how to use it properly and safely and how to bypass pumps and boilers etc...