Just shows the full pinned messages in the sidebar. That's it. Mostly useful for if you're on a free Slack, where messages can be pushed out of the visible history, but stay pinned. In that case, you can't see the full message, and can't click on the message to go to it to see the full thing - but the full contents are only hidden with CSS, which is easy to "fix".
Or just click on the Raw
button below, above the css file. That works too.