to go into more detail;
its your carbon cannister. whether you need it or not is up to you. however it does serve a purpose.
when your fuel tank is cold, everything is fine. but if you hit a 30 degree day the fuel in your tank expands and needs to escape. the main place it will escape from is your filler neck (where you fill up with fuel) and on older cars on hot days you can actually see fuel vapour evaporating from the neck.
now here is the tricky bit. wet petrol isnt really that flammable. dudes in the movies drop cigarettes in petrol and WHOOSH up it goes... but its all bull. the stuff you wanna avoid is fuel VAPOUR. that stuff WILL go up with a bang.
what the cannister does is filter the fuel vapour and turn it into O2 and water (the hose out the bottom drips the leftover harmlss stuff) so when pressure builds up in your tank, rather than escaping out the filler neck it goes to the cannister and filters... thus making your flammable wreck a lot safer.
having said all that i took mine off and plugged the hoses cos its ugly., LMAO.