Safety group without sewage connection?

In a used house, a hot water boiler is to be installed. There was already a boiler in that spot, which the previous owner took with them. The plumber wants to install an additional safety group (see photo). However, without connection to the wastewater system. I assume that the siphon will then be omitted and simply closed off at the top. What disadvantages would I have from this? Where does the water go that is created by expansion due to heating?

Best regards Egon

If I understand correctly, when the boiler is "completely full" cold, the heated water should flow into this expansion tank if it is not needed, so that the then "closed" boiler is not burst. When it boils, it is no longer water at all, but steam (which would not escape through a drain pipe). The boiler cannot be more than "already completely full when cold," and the water cannot expand in liquid form more than until it fills the expansion tank. A boiler is emptied by drawing off the heated water, with the expansion tank communicating with the main tank.