Most of the time helpers act as sensors or variables, but you need a way of assign a value, via configuration or automations, based on the state of other sensors. In this case, having a template where you can include the required operations would be very useful, and reduce the need for other automations.
Can you clarify how this would be different than the triggered variations that already exist for template Sensor, Binary sensor, Number, Select, and Buttons?
Actually, I was thinking in something similar to state based template sensors, but defined through the GUI instead of tweaking the configuration yaml.