There were other names done in this way as well, resulting in Edw., Jas., Jo. (another variant of John), Jos., Nath. (Nathaniel), and even Da. for my own name (however spelt).
The way in which these abbreviations came into use makes perfect sense. Being in a job where I have to sign scores of purchase orders, timesheets, parts releases, etc. each day, I long ago shortened my first name signature to "Da.", quite unaware of the origin of the wider practice but for the very same reason.