TVTropes doesn’t have anything listed as the origin, but it has a ton of examples if you want to go back in some of the media and see how far it goes back.

The reasoning they give to its popularity is that floating text is easy to animate, can be very flashy for little effort, and can be used as a plot device, with either characters needing time to draw runes, or people refining/debugging their spells, and having something observable to show off the magic power level or the effective range of spells. Basically, it’s a really cool, easy, and cheap way to make magic look badass.