There are so many different mechanical faults that can arise on a particular car and an unusual sound can be the first warning sign.
If you really listen to your car from time to time rather than just cranking up the stereo, then you may be able to identify part of the problem, which makes life a whole lot easier for your chosen mechanic. Wheel bearings can make a grinding sound when they begin to fail, whilst brake pads will also grind or screech when they need replacing.
Power steering will vibrate when turning if there is a lack of fluid, and exhausts can really roar and blow when they’re damaged or split. An unusual sound may well be nothing, but it’s always better to get it checked out early, so make sure you’re listing out for anything peculiar on a regular basis.