Adding on to eyegor's response, in some instances, a thin layer of sediment can build up in a gas tank as well. This is normally not an immediate concern in and of itself but letting your fuel pump run too close to empty can start drawing in that sediment which can eventually clog the pump. It's worth noting this is more common in vehicles that tend to sit for a longer periods of time, like lawn mowers, motorcycles, and grandma's beige-on-beige 2008 Camry.