what does the evap system do ?

i have a 2001 ram 1500 with a 5.2 liter 318 . i was wanting to know what the EVAP system do and do i need it !!

Your EVAP system takes fumes from the gas tank, mixes them with air then feeds it to the motor while it's running. It keeps raw gasoline fumes from getting to the outside air...a pollution consideration and it is unwasted fuel when it's put back in the motor.
It does not hurt power one bit.