That is usually to avoid bots. Any normal person can easily get a couple hundred karma by just posting a bit or commenting a few times, but any bot doing the same is gonna undergo scrutiny.
Bots just repost comments that were popular in the past and get huge amounts of karma, then delete said post before anyone catches on and starts and downvote wave.
If you have less than x amount of karma on a good portion of subReddits you can’t post at all.
That is usually to avoid bots. Any normal person can easily get a couple hundred karma by just posting a bit or commenting a few times, but any bot doing the same is gonna undergo scrutiny.
Bots just repost comments that were popular in the past and get huge amounts of karma, then delete said post before anyone catches on and starts and downvote wave.
That system is gamable and solved.