Why replace an oil boiler nozzle?

My Harcal 430 boiler has been running fine for well over 20 years with the same nozzle. It started rumbling on shut down and as I then needed to replace the solonoid valve, I decided to replace the nozzle. No noise on shut down, but it then began starting with a 'thump'. Thought at first that it was trapped air, but no. Another new nozzle (different make), same problem. Eventually replaced the original 24 year old nozzle. Result perfect! Why does everyone think that the nozzle needs to be replaced every year?