Just as a side note, there is really kind of a variety of things that (at least sometimes) go under the umbrella of 'long-lining'.
There's ground-driving, where the trainer is generally back out of kicking range, usually with the lines run thru a surcingle but sometimes run thru stirrups (up or down) or even sometimes just hanging free from bit to trainer's hand. The different setups have different strengths and weaknesses, and different sorts of hazards - for instance, with reins run through a surcingle it is frighteningly easy to completely accidentally pull a horse over backwards if it spooks, turns round and starts backing away from you (you need to go with it, *not* pull on the lines), but having the lines free from bit to hand poses a higher risk of the horse getting hung up in them if he does something unexpected. Etc.
There is also the more dressage-style (tho it is not done *only* in dressage) where the trainer walks right behind the horse, closer than in ground driving, close enough in some cases that you could rest your hands on his haunches. Obviously you don't do this til you've worked out any kicking issues
Again, there are variations with different sorts of surcingles or with nothing between bit and hand, for various reasons and with various pros and cons.
Ground driving on longer lines has the main advantages (IMO) of helping make the horse bolder, and providing a nice stepping-stone to being ridden. Obviously it's also good preparation for driving
Working closer behind the horse is generally done for more precise training goals, such as working on balance, engagement, control of the haunches, lateral work, that sort of thing. Not to imply it is not useful for young horses too.
Also, some people use 'long-lining' for what personally I would call double longeing, where the horse is kept on the longeing circle the whole time, but with a second longe line running to the outside bit ring (more rarely, caveson ring). I personally have never had much luck with this but then I am afraid that I am not especially great at longeing beyond the basic 'spin the hunter around to tire him out' sort that I learned as a kid
Pat