Services gather events data. For example, the Syslog server service accepts incoming Syslog messages and the Event Log Monitor extracts Windows event log data. There can be unlimited multiple services. Depending on the service type, there can also be multiple instances running, each one with different settings.
You must define at least one service, otherwise the product does not gather event data and hence does not perform any useful work at all. Sometimes, services are mistaken with service defaults those are pre-existing in the tree view. Service defaults are just the templates that carry the default properties assigned to a service, when one of the respective type is to be created. Service defaults are NOT executed and thus can not gather any data.