Pipe aerator necessary in the bathroom

Hello everyone.
We had our planning meeting with our installer yesterday.
It was now the third appointment and suddenly in the final offer a "pipe ventilator DN 70 for installation in pre-wall washbasin system" for €250 appeared.
This is the first time we've heard of this; it was never mentioned before.
Now our question is, is this pipe ventilator really necessary for €250? I have seen on Google that they usually cost around €50, which is a huge difference.
Unfortunately, we have never heard of such a pipe ventilator and have no idea if it was still necessary in buildings from 2017.

Thank you very much

Best regards, Benkler

With a ventilator despite a window, I think of a connection with an automatic ventilation system (?)

I would definitely recommend it to you, because otherwise the water in the downpipe will empty the trap and you'll have a lovely smell in the bathroom.

If the plumber thinks that the minimum necessary roof venting will not be sufficient ...

When I think of an aerator despite a window, I think of a connection with an automatic ventilation system (?)

You are on the wrong track... two different issues...

@OP
If the plumber says it is necessary... then you will probably have to bite the bullet... if you want to have a warranty on the whole thing...

P.S. Most of these things can be had for well under 50... you can try to negotiate with him...

If the plumber thinks that the minimally required venting through the roof will not be sufficient ...

Exactly the venting through the roof will be missing for him, that is the problem.

If it is no longer (or not) feasible, you will have to bite the bullet.