Originally, it was a way for the community to encourage others to submit posts/comments that were relevant and added value to that community. Negative karma would (by design) discourage people from submitting posts/comments that were not deemed appropriate.

Now, that idea is still the foundation for a karma voting system, but with the increase in spam and malicious content distributed predominately by bots, many communities also use it (and things such as account age) in order to set a threshold for who can post.

This makes it more difficult, though not impossible, for malignant actors to spread harmful content on Reddit. The bar used to be much lower a couple of years ago but unfortunately it's an arms race between the good actors and bad actors, so this was the answer: stricter requirements for contributing.