Aitch, you need to get one (a roof box that is) not only can you put shed loads of crap up there is also will add to the overall camping experience.
You see when you have filled it up all beautifully and neatly BEFORE you try and shut it then you have a little smug feeling that quite possibly you would have done very well on that bit of the krypton factor where they had to shify blocks about to make a nice, orderly pattern.
And then you try to shut it and the little fucker never shuts and you can't lock the bastard until it is fully shut. So then you rearrange and rearrange and attempt to shut it over and over and then you fall off the car back seat and bang you chin
AND THEN YOU FLY INTO A BESERKER RAGE
and you just ram things in, slamming and slamming and the blasphemy is something just terrible-muriel.
And then it shuts.
And this is all good you see because when you are trying to erect your tent in the bastard syphillitic rain and your dd has pissed in her pants THEN you look back on your trials with the shit-head roof box, and it all doesn't seem so bad after all.
And they are useful for putting stuff in.