Well, there's two big changes. One is that this allows a cleaner separation of concerns. Using inter process communication (IPC) we now have a framework for splitting out things into microservices. Under the hood this is a capnproto server and client that dispatches requests directly into our c++ code. For now there is just a gui and a node service, but in future we will have a wallet, an index, a mining, and maybe even a p2p and mempool service. The other thing it enables is a fully programmable interface for external devs to work on. You can now create your own very performant GUI for Bitcoin Core that just uses IPC in a language of your choice as long as it supports generating capnproto schemas.

We also expose a mining interface in this release that Sjors used to hook up to a sv2 template provider in his own binary, so it is also a big step towards making Bitcoin Core usable with sv2. But it might also be interesting to use as an alternative to the current getblocktemplate rpc in general.