Read this: Sonos - the multi-room audio missing manual
This turns your smart speaker/soundbar into an intercom-lite system with other speakers around the house. Just say "Alexa, announce that breakfast is ready" and it will play a chime and then the message on all other speakers - especially handy for larger houses.
It's not actually an intercom because there's no two-way audio so your family or flatmates can't respond via Alexa. Google Assistant has a similar feature for Home named Broadcasts which similarly doesn't support two-way audio.
It looks like Announcements will work across Sonos and Echo if you have a mixture of devices but we'll confirm this once we've tried it out. In fact, features like this might be a reason to plump for an extra Amazon Echo Dot if you've already kitted out your main spaces with connected Sonos kit.