Why do you do it?

It's common among trolls, not among the vast majority of users.

Not just trolls, there is a giant incentive for marketers (or even just TikTok-ers) looking to make something visible.

Tons of accounts reposting content from ~2 months ago to create a long, legitimate-looking post history and high karma score. They then use those accounts to complain about Product X or love Product Y -- and they don't look like a shillbot. Ditto for political actors, propagandists, et al.

Trolls, professional marketers, reputation management firms, social media "influencers," and even just social media "power users" who don't rise to the level of "influencer" are all known do this. This is a vast minority of users thanks to the Pareto principle and power laws. However, I suspect that the majority of posts you see on the front page of pretty much every social media app have had some degree of "help" from things like this.

One Reddit user, Unidan, was famously banned for doing this with 12 accounts, and in retrospect, this sort of scheme may have "made" his success - his replies showed up as the top comment a lot (even when they were not insightful) and it got him significant name recognition.

A lot of morally dubious people, maybe.