More robust. You can use P2P stuff offline in a disaster scenario. The entire thing will never be down for maintenance or from an error.

I haven't yet familiarized myself with this particular protocol, I have something like Briar in mind when it comes to P2P chat.

Why do we need decentralization in any app?

Short answer; When done well, it makes the app more resilient against a variety of issues/risks that arise from relying on a server/service/single-point-of-failure.