It provides sentiment without clutter. Instead of 1000 identical comments or 1 comment with 1000 "same" or "this" comments underneath.
I admit that seeing my comments get upvoted gives me a hit of dopamine, knowing that that number of people were interested in what I said.
However, I never look at my TOTAL karma, just each individual comment. I like knowing that THIS thing I said people agreed with.
There is a whole host of behaviors on reddit around acquiring user karma.
For individual post karma, I'm the same as you.