It's all very complicated and has references to the Bernoulli principal and crap like that.
In short, I am informed by Rob at Needswings that the big hose ( the one at the back on the left side ) is to equalize pressure in the engine, the 2 little ones ( that go to the catch can ) are to suck the bad old oil fumes out of the engine and poke them back through the throttle body so they can be burned. The catch can stops that.

I got real smart one time and decided I didn't need the big hose, so I capped it off at the cover and the air intake pipe. Bad mistake. The engine made noises you would swear was a water pump going bad, it got all happy again when I replaced the hose.

N'es pas ?