I think that it's mostly for reputation. Accounts with high karma mean that a person is an active user, and constantly provides valuable, funny, etc. content to others.
Reddit karma is similar to a user's score in that it adds up all of their upvotes and subtracts all of their downvotes. It offers a few practical perks, such as building your subreddit and joining some exclusive communities, but it's mainly about reputation.
Reddit karma is basically the lifeblood of reddit. The more you get the higher your posts will show up attracting more people. Bad news is that a lot of karma is being produced by bots these days so I tend stay away from reddit for that reason :(
It can help you pass through automated moderation.
Some subreddit requires at least 200 of karma to post. There is also a threshold to include links in the post.
It gives you a few perks but it's mostly there for credibility and the reputation you have in the community. People that aim to be subreddit moderators should go for higher karma as it makes them known and trusted in the community.
It's a replacement for the algorithms that other social media sites use. Instead of controlling the content and where it ranks, the audience does that. The danger is that higher ranked comments get more visibility and likely get more upvotes, so it can be self-serving cycle - especially if a comment gets in early, it can be biased towards the top.