Pipe aerator / backflow preventer leaking - why?

Good day dear community,

We moved into our new building a few weeks ago and have the adapter shown in the attachment on the cold water connection in the kitchen (and only there), I think it is a pipe aerator with a backflow preventer.

I have connected a Y adapter behind it to connect the dishwasher and the fixed water connection for our side by side.

My problem is that this adapter cannot be sealed, I have tried everything, without it everything is dry.

Can someone tell me what it is for and whether I absolutely have to leave it on?

Thanks & best regards

Yes, as far as I know DIN 1988.

I believe the thing forms an air layer when no water is flowing. This is necessary so that the water does not accumulate germs or contaminants do not flow back into the water pipe. I had to retrofit something like this on faucets that are not used permanently.