Car won't start, you got in an accident, or you made the mistake of parking in a private lot in Bethesda. Towing is one of those things where you usually don't have time to research - you need someone in 30 minutes, and you have no idea where your car ends up.
The biggest problem with towing is you're at their mercy once they hook your car. You don't know where it's going, what they'll charge for storage, or if that 20 minute ETA means two hours. It's getting better, but you still need to know what to watch for.
For non-emergency situations like moving a non-runner, shop around - rates vary from $75-$200 for local tows. For accident tows, your insurance company usually has preferred providers. Get the destination address in writing before they hook up your car - once it's moving, you have a lot less leverage.