Bump is a signal that the version you're pushing is stable and the users should upgrade to it.
When your extension is first referenced, the latest bumped version is used. Similarly, if there is a newer bumped version, a little upgrade button will appear next to the extension.
Commits without bump are generally not accessible to most users, so they are mostly for you to keep track of things.
In order to appear in the Extensions dialog -- or in search result, you need to get your Extension approved. In order to get approved, open an issue on GitHub at https://github.com/Microsoft/pxt-adafruit/issues.