Although there could be any number of reasons to use an alias that a person can think of, like maybe hiding a criminal past, avoiding bill collectors etc, I know of one specific real case where a boy was orphaned and sent to live with a guardian who used him as an unpaid apprentice, I think making shoes. He hated the work, and didn't get along well with the guardian. He finally ran away to enlist and was accepted as a recruit. The guardian advertised him as a runaway and found him and brought him back. He then waited a few months and ran away again this time choosing to use the name of a friend so if the guardian sought him out again he would not be found.
He was underage, and since his "parent" would not sign his permission, he lied about his name and age and enlisted into a state volunteer infantry. After the war he stayed on in the regular army still using the same alias since he was already known by that name within the military. Later as a civilian he worked near the army fort out west as a contractor or wagoner or something, and still kept the false name.
Finally when he married he switched back to his own name so he could raise his children to carry his own family name. As an old man he hard a very hard time collecting his soldier's pension because he had to prove he was indeed the same person who served many years before.