Hi all

In our shower waste arrangement we have:

A chrome coated plastic grill.
A removable cylindrical part with a cross bar that locates in the trap body.
The trap body itself.

At cleaning time (both wife and daughter have long hair) I remove the chrome
grill and twist/lift out the cylindrical part.
The latter usually has a lot of hair caught on it which can be easily
cleaned off.
The thing is that between this cylindrical part and the body of the trap
there is an 'O' ring and I can't figure out what it does.
At some point the 'O' ring has become stretched and therefore fouls the trap
body. I have removed the 'O' ring for now to allow the cylindrical part to
be re-fitted. As the cylindrical part is open at top and bottom, I can't
work out what the 'O' ring seals IYSWIM.

Any ideas on the purpose of the 'O' ring?

TIA

Phil