The line that runs from throttle body to the rocker cover/fill point. what is it for and what is it used for?

Only thing i can think of is supplying a return for pcv valve but still that doesnt even sound right. im only takling a stab in the dark.

Any yhelp would be good as if i dont have to put it on i wont, as with the new rocker covers i will have no wheer to put it unless i make a fitting up.

cheers
Ben