My wild guess would be that it started as a composition character on typewriters to write things such as ã. For those unfamiliar with how accents work on a typ

My wild guess would be that it started as a composition character on typewriters to write things such as ã. For those unfamiliar with how accents work on a typewriter: There'd be a couple special accent marks such as ~ and ` and whatnot that'd put down their accent but not advance the carriage. You could then hit a letter to get e.g. à or ē. It's a pretty clever system to prevent having to have a key for every accent.