Air-water heat pump - expansion vessel

When I was standing in the utility room today, I was slightly shocked. Suddenly, the heat pump was already there, BUT next to it under the ceiling, a "vessel" was mounted. That actually shocked me even more. Since in my previous research I had never read anything about a vessel in connection with an air-to-water heat pump (only seen in connection with a gas heating system), I wonder, could this have been avoided? Do you have something like this too, or to what extent does this - as I then found out - expansion vessel help me?

We have something like this too but it is not a split unit in the heat pump and ours is about 80x80x200cm so there is enough space. But it is necessary.

Heating water expands when heated. If you want a warm house, you need various parts for the system, including an [MAG (expansion vessel)].

All right. Thanks, everyone!

you probably have two now, in your air-water heat pump there will also be a small vessel. We also have two, worse things exist.