For competition cars it's a legal requirement (stops oil getting on the track).
The idea is that race engines tend to produce a lot of blow by gasses (relatively loose ring/bore clearances). These pressurise the crankcase, so you tend to both weep oil from the seals and dump oil mist into the inlet tract and cause detonation.
So, fit bigger breathers, route them to a catch tank, and all is well (sort of).
To be road legal you still need a hose to the inlet tract so they don't vent to atmosphere, but hopefully the catch tank will effectively remove most of the oil vapour.
The high boost tubs do blow a lot of oil.
Lots of other tricks out there -- dry sumping, oil scrapers, crankcase evacuation pumps. All nicely discussed in the A Graham Bell 4 Stroke Performance Tuning book.