ᐅ Pressure relief valve assembly without wastewater connection?

A hot water boiler is to be installed in a previously owned house. There was already a boiler in that location, which the former owner took with them.
The plumber wants to install an additional safety group (see photo), but without connecting it to the wastewater system. I assume this means the trap will be omitted and simply closed off at the top.
What disadvantages could this cause? Where does the water go that results from expansion due to heating?

Best regards
Egon

If I understand correctly, when the boiler is "completely full" and cold, the heated water flows into the expansion tank if it is not needed, so that the closed boiler does not get damaged. When it boils, it is no longer water but steam (which would not escape through a drain pipe). The boiler cannot be more than "completely full when cold," and the water cannot expand in liquid form beyond filling the expansion tank. A boiler is emptied by draining the heated water, during which the expansion tank communicates with the main tank.

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