With seasons, if you're in trouble you're only in trouble for a short time, then you have three months where you can direct your attention somewhere else and attempt to rectify the problem before it reoccurs.
Without seasons, you can't do that. Your realm is collapsing right now, and it won't give you a breather. You don't have time to go declare a war to grab more rural regions and bring them up to shape so they produce enough food to feed your city - it will have starved before that.
But with seasons, a well-timed raid on your realm will starve it when it otherwise wouldn't have. Or a drought in the wrong regions at the wrong time. Seasonality also increases rot and bring a greater need for investing in granaries.
All in all, seasons make it more important for realms to care about food, and harder for them to plan ahead for it.
Plus, we now have rations anyways, so a realm with food deficits can adjust its cities' consumption until it fixes it.
