The same can be said for corn, meat, paste, maggots etc etc. its down to the individual. Hair rigging can give you a superior bait presentation as well as improving the chances of a successful hook hold. Added to that, as Alan said, some fish react differently to different presentations and chub can be spooked off your bait by putting bait onto the hook or by using a longer hair.
Its all just little bits of the big picture. If you have any doubts about the benifits and mechanics of hair rigging I suggest you read some carp magazines or have a look at one of the freebie Korda dvd's available in your tackle shop, it will open your eyes! Fortunately barbel feed like a vacuum cleaner with a very underslung mouth which makes them a lot easier to fool than the average carp which is why a hook loaded with casters will still work.
If I want to hair rig casters or maggots I put a blob of superglue onto my bucket lit, lay the hook and hair out on the lid and for maggots, I just dip their tales (the fat end) into the glu then press them onto the hair, after a second or two they will stick. Do this to as many as you want and you will have a writhing mass of grubs, irrisistable to barbel and just about every other fish.
For casters you can glue three along a hair like a little triangle or just randomly stick them along the hair so that they look very natural a presentaion that also works well for small pellets.