To add to Rock hills explanation. The tow company has a contract. They may have contracts with several ERS, police, the local mall, etc. They service emergencies first. After that they take calls pretty much in order. But not all of their contracts pay as much or as quickly. So the closest and best paying calls are often given priority. The more calls they can make in any given shift and the better and faster the contractor pays them plays a big part in the response time and service.
The tow company doesn't care if you call everyday for service, they get paid no matter who calls them. However your ERS monitors your use as it does matter to their bean counters.
Some ERS companies have a reputation for slow pay or trying to negotiate service calls after the fact. These companies will have less reliable and forthright tow companies.