The rocker (cam) cover breather system pipes which comes out of the the top of the engine are connected to the induction system, under certain conditions, particularly on higher powered engines and engines being soundly thrashed, oil can be drwan up the pip and into the induction system. The reason being this prevents oil fumes, vapours, bits of oil or sludge entering the environment straight out. By entering the induction system they are combusted and then passed out through the catalyst which is cleaner than oil fumes and oil just spewing straight out to air.
Sometimes, particularly on a knackered or high powered motor, oil sludge can effect injectors and line the inlet. There are catchment areas to prevent it entering the engine, but it does nto always work.
The breather intake on the inlet can be blanked off to stop it sucking unfiltered air in and the crank case breather can go straight to a little air filter, this was popular on Minis, where the timing chain housing had a crappy outlet which went down a pipe into the carb.
Supposedly a small performance gain is available as the fumes entering the inlet or top hat setup on some XE engines, can cause the flow to be messed up.
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