If it wasn't being used as a means for ego, there is monetary motivation. During the migration off reddit there was some discussion in the fediverse about how/where you could sell your reddit account and for how much depending on the karma.

Why are legitimate accounts valuable? Basically, if you're a company and you want to shill your products without spending money on ads, you can do "native marketing". It's basically spreading word about your product through normal comments and posts on social media, attpting to blend in with natural discussion and legitimate product reccomendations by real people. There's also some studies that indicate that people are more receptive to this sort of thing than more traditional marketing.

It's easier to masquerade as a real person when you can buy an account in good standing that has "real person" posting habits from before it was sold to you.

Like most subreddits it devolved over time, but hailcorporate used to be pretty good at calling out this sort of stealth marketing, like when posts would make it to the top of the picture based subs with product placement in the background. Like "I thought I could trust my new puppy home alone finally, but I came home to this" picture of a torn up couch, but the coffee table has fast food bags on it placed with the logo clearly visible. Sometimes they even reported groups of shill accounts by documenting coordinated changes in posting behavior across groups of accounts.