What can I do with the karma I've accumulated in my Hacker News account?

This [1] explains what can be done at different karma levels. Otherwise it's just a number. I ignore my karma using uBlock in My Filters.

    news.ycombinator.com##.comhead .score:style(overflow: hidden; display: inline-block; line-height: 0.1em; width: 0; margin-left: -1.9em;)
    news.ycombinator.com###hnmain > tbody > tr:first-of-type table td:last-of-type .pagetop:style(font-size: 0!important; color: transparent!important;)
    news.ycombinator.com###hnmain > tbody > tr:first-of-type table td:last-of-type .pagetop > *:style(font-size: 10pt; line-height: 1.45em;)
    news.ycombinator.com###logout::before:style(content: "|"; padding: 0.25em;)
    news.ycombinator.com##form.profileform tbody tr:nth-child(3)

[1] - https://github.com/minimaxir/hacker-news-undocumented

You can put them on The Blockchain and the write smart contracts to trade your HN karma tokens.

Revel in the "number go up" ness of it.

Screenshot it. No one will believe!

burn it by writing something true, relevant, and/or useful