Say you want to build a Peer to Peer application; chat, file sharing, music sync or whatever, then something needs to be built to communicate between this application running in two different places. While you still need to build the actual protocol yourself ("Users can send messages" etc), how the two instances are connected is handled by Iroh mostly.

Not a stupid question - I was wondering about the same thing, and this gave me easy access to the answer because someone replied to your comment. Thank you for asking :)

With tailscale, you can establish a private LAN over the internet. Iroh embeds this directly into the app itself.