What is the history of souvlaki and is there any benefit in cooking meat on a skewer?
Historically, the practice of skewering meat had several advantages over building a rotisserie and cooking a single large piece of meat. Think about the advantages of cooking over a primitive camp fire, rather than a modern grill:


Cooks quicker, saving time but more importantly fuel for the fire (think desert or other difficult terrain)
Did not require plates or forks and knives to carry around if you were nomadic
Still cooks over a spit so can avoid falling into the coals


These days there are some other more modern advantages like:


Can use cheaper, tougher cuts effectively
Marinates/brines faster
Makes it easier to turn or flip on a grill
Skewers are fun


Puncturing the meat prior to cooking does not cause a large loss of meat juice during the grilling process. Maybe if you removed the meat from the skewer immediately after taking it off the grill you would lose more juice than had you never skewered the meat at all, but it's not like the original puncture will expose the center of the meat and allow everything to spill out. Leaving meat on the skewer to rest will be as effective as grilling a single unpunctured piece of meat.