The boiler control utilizes T&T thermostat terminals, and this requires a dry-contact to open/close the control circuit to engage the boiler. Typically, boiler controls require only an open and closed contact switch, therefore an isolation is used to separate the 24VAC that is returned as a control signal from the ecobee thermostat.
This isolation relay acts as the 'required open/closed' contact. We refer this to a dry-contact and is quite normal for various manufacturers to use this method.
A separate 24VAC Transformer and an Isolation relay is needed for this type of application.