Why does the cyclone breather exist?

So I know that the cyclone breather is there to seperate oil vapour from the air and return it to the sump but what I want to know is why does air need to get out of the engine and back to the airbox?

I am just a curious person and I am interested in why things work rather than just how they work! Sorry to be so dim but whay does air need to come out the engine and back to the air box??!!

years ago engine breather would have vented to the atmosphere ,but because of emission worrise its now but back through intake and combusted cyclone stops air filter getting oil logged like on 2.5 na and td engines,engine produces pressure in crank case from combustion pressure leaking past rings ,and in the case of 200 tdis and earlier engines through vac pump that sucks from servo and exhausts in block

So how important is it that that little rubber seal is really tight? Presume the worst is that oil vapour leaks out or crap gets into your engine but it does not need to make an airtight seal right...

Thanks for the infor James! Its interesting to me.

which rubber seal

Where the cyclone breather goes into the engine block there is a little rubber o-seal on the clcylone breather pipe to give it a snug fit. Just wondering if this has to be really airtight or if its ok to just be a good fit.

When I put mine in i smeared some silicone around the neck and seal so as to stop any oil leaks - possibly not necessary, but it made me feel better

i use a bit of silicone to help ,if it doesnt seal it leaks oil