What is the purpose of a oil catch tank and if it benifits the car?

 

part of the clue is in the name.... 'Oil Catch'

From the factory engine breathers are plumbed back into the inlet. Oily vapours can reduce octane. Worst case you get a good squirt of oil up the breathers on hard acceleration/cornering and it goes into the combustion - giving a nice james bond style smoke screen.

The catch can catches any oil. There are a number of options after that. Keep the oil in the tank, or return to sump. Vent the fumes to atmosphere (Tree huggers will be after you!) or Vent the catch can back into the mixture. The later will avoid potential smells in the cabin.