« On » is derived from the latin « homo », which means human being (in general), and by extension : anybody.
« Homo » is a noun, and was thus preceded by an article. The Bible provides an example :
« L'homme ne vit pas seulement de pain. »
« Men don't live solely on bread. » (free translation)
Later, « homo » evolved into « on ». It became a pronoun, but an old trace of its nature as a noun remained.
This is why you come across « l'on », mostly in written texts. « L'on » means « L'homme », i.e. anybody.