Statefulness is a pain in the ass. But it brings with it enough value to keep people interested in maintaining state. And really, everything is stateful, even things that claim to be stateless. How so?
There are two kinds of state: peer state and configuration state. Peer-state is what you hold in your arp and routing tables, and it is what load balancers maintain in their connection tables, and it is what applications call state. It is event-sourced data that comes in at a ratio of or near 1:1 with packets or sessions. Configuration state is the start conditions of the system modified over time at a rate of 10:1 or higher to packets or flows. Modifications should also be event-sourced. The difference between the two is frequency of change.