The only thing stopping you from ramming the passenger van that just cut you off is your brakes so these are probably the most important factor when it comes to DOT inspections. At every annual inspection brake drums are removed and measured to see if they are still within spec, brake shoes are also inspected for cracks and uneven wear, also they are measured to determine if they are in spec. They are also inspected at every PM inspection. If your brakes are squealing or your wheel seal is leaking all over them it can cause big issues on the road. A leaking wheel seal can cause a fire if it’s bad enough, so it needs to be dealt with immediately.