Over the years your computers cooling fans suck a lot of dust through your case/tower clogging the cooling system and even making your fans noisy.  The fan that sits on the cpu heatsink can clog the heatsink causing the main processor to over heat or just run hotter than it should.  This can make your system not as stable as it should be.

Often when I open a case there is a thick layer of dust sitting on the bottom of the case.  This has no negative affects on the computer other than a ready supply of dust that can end up on the fans.

The cleaning service is not that expensive so why not get the inside cleaned.