Sometimes you're announcing something: a tour, a song, a video, a collaboration, a new merch line.
Whatever it is, it probably doesn't need a lot of words. Having the announcement on a website makes it well indexed for search engines, and since you have full control over the experience you can add any relevant links or embeds to make your statement really resonate.
If you choose to use an announcement instead of a poster, just know that they're thought of us more of a continuum of each other, and you link over to the next one (see below) as opposed to the posters who are stand-alone.
It also means if you choose to use the Articles module on any of your pages, all public announcements will be listed in a browsable feed.