Coffee is by nature a very oily substance.
Over time, coffee oils accumulate inside of your machine to form a rancid residue. This residue is responsible for the acrid smell you may recognize coming from a poorly maintained machine.
Unfortunately, this residue causes more than just bad odour.
If allowed to develop, residue inside of your machine will eventually clog its filters and tubes and cause it to malfunction.
Well before your machine clogs or you even notice the signs of neglect, the stale residue lining the inside of your machine will begin to affect the taste of the fresh coffee you prepare.