Roadside assistance is generally reserved for those a part of a membership club. In Australia it is RACQ, you pay a yearly fee and get roadside assistance when you're broken down. If you're not a member, you can get assistance but have to pay (especially if you need to be towed).
Much like Uber revolutionised the taxi industry, this app could revolutionise roadside assistance and towing. You won't need to pay a yearly fee, see roadside assistance vehicles near you and get a flat rate call-out and fix estimate price.
Mechanics could install the app to make some additional money on the side. During quiet periods you could send out a mechanic to help someone broken down, marketing this towards mechanic shops to make some extra cash could also work, but pigeonholing it to a tight segment of the market could limit its spread.
Uber is an obvious existing solution as well as preexisting roadside assistance services, there are a few other little clones around, but if done correctly, this could be a market leader in a relatively untouched niche.