Accounts involved in obvious karma grabs often show up sometime later with product "recommendations", noting that they are an established high karma account and definitely not a marketing person.
It's worth mentioning Reddit uses the Karma system as part of the rate limiting. If you have a lot of karma you'll virtually never get the dreaded "You're doing that too fast" error message, but if you have a low karma account you might only get to post/comment once every 30 minutes.
It doesn't signify much as you say, but there is a minor industry of farming reddit karma and then selling those accounts for use by people who want to astroturf.
Not everyone is logical in this world. Karma is like dopamine inducer to many people. High karma might make somebody feel good. It is just like a game where reaching high score generally do nothing but you know that some people want little bit of satisfaction ..?