For music in the car, some folks tether their iPod or phone to the auxiliary jack, but that is a poor choice as it means touching a handheld device while driving.
Over the last 10 or 12 years, car manufacturers have seen the need for MP3 audio integrations through the factory car stereos. The car stereos are much more capable than ever.
Early attempts at MP3 integrations were weak, and had very specific USB hardware support, resulting in stuttering audio or USB device restarts, playing music from the beginning, starting at track 1 again. Vehicle restarts would start at track 1….AGAIN! Random, being not really all that random. These quirks are really frustrating and somewhat distracting for the driver.