Telecommunications seems like the biggest thing. I think agrarians tend to be spread-out, so any communication tech faster than horse messenger would be hugely welcome.
It's not just about convenience, either. Consider that even if food and supplies must be hauled overland slowly, coordination is itself a problem that flows from glacially-slow information transfer. This is a society that has seen starvation because information about unexpected food or supply shortages wasn't disseminated quickly enough to be actionable.
And because so much of their economy consists of perishable goods, timely and accurate information is vital. Whenever there is a mismatch between what gets shipped and what is needed, lots of people suffer. In the best case, surplus goods are sent to where they are not needed, and so the people who produced those goods lose their investment. That by itself can lead to major hardship. In the worst case, unanticipated needs are not met quickly enough, and lives or crops are lost.
Oh, and even though they're agrarian, it's not like food and farming are their only concerns. War, politics, crime, disease, natural disasters, predators -- wherever there are people, these problems are too, and people need to talk about them. They need information about the facts, and (unless they are all at war with each other) some will want to coordinate their responses.
The Western genre can only exist in one place: the boundary between wilderness and civilization. One of the clearest harbingers of the coming of civilization -- the eradication of the wilderness -- is the telegraph. A society without the telegraph is still the wild west.
Throw in a steam engine and you're practically living in post-WWI America.
ETA: second place might be electric lights. Electric lights are superior to fuel lamps for a few reasons (including that they don't spew toxic smoke directly into living spaces), and so electric lights can be adopted more widely than lamps. Artificial light allows diurnal creatures like us to be productive for longer each day. I imagine one of the biggest impacts is just that it means farmers suddenly have more time to read, which means education, which leads to literally all the good things. Education is the closest thing humans have ever found to a silver bullet.
But even if you don't want electricity to let them bootstrap their way to smartphones and credit-default swaps and dark web chat bots, it'll make a big difference simply being able to do the paperwork and read the latest farmers' almanac after dark, instead of those things competing for daylight with the farm work.