Dijkstra said that it was essential to get your knowledge out of your own head so that it wouldn’t die with you: to transmit it to other people. But I’m selfish enough to find that less compelling than several other reasons.
One is that, when your knowledge is only in your own head, it’s easy to fool yourself into thinking you know things that you don’t really know. If you try to tell other people about it, sometimes they will be harder to convince. They may need to see more convincing evidence than the evidence you had gathered previously. By organizing the evidence and gathering more of it, you may discover that you were mistaken, in whole or in part; objectively observable evidence is generally higher-quality evidence. Sabine Hossenfelder wrote a very interesting article about her time listening to physics crackpots expound their theories, and in many cases they didn’t have a clear idea of what kind of thing would count as evidence, or even as a physical theory.
Another is that they may offer contributions: they may suggest that the technique you have devised would be useful for something you hadn’t thought of, or point out a weakness you hadn’t seen, either because they know something you don’t or just because their perspective is different from yours.