The same thing it would do in a multiplayer game. The server manages the core of the game, while the client things that do not affect the game, like UI or listening for key presses.
It is actually easier (as in less chances of bugs) to develop games this way. Not to mention the best side effect being ready for actual multiplayer in the future.
Minecraft does this too. In the past when it was a single process, they had to actually develop two versions in parallel (one for single, one for multiplayer), there was always bugs and even differences between versions, like some machines would work in single player but not in multiplayer or vice versa.