Harvest time can be make-or-break for farmers. Stuff you don't expect like a large storm at the end of summer can blow over corn stalks. That makes it a lot slower to run the combine at harvest time. If early fall storms are forecast you might literally need to get everyone available out in the field to harvest the corn in the next 3-4 days or you can expect the value of your crop to drop dramatically - everything will be wet and it's no longer warm or sunny enough for it to dry out. The field turns to mud and the grain elevator ain't gonna pay full price for water-soaked corn.
Everyone else is in the same boat so every available truck+driver is booked. Every mechanic is already 5 tractors backlogged from all the other farmers with emergencies.
Then your harvester/combine dies.
If you're with JD... they have a crews of engineers/mechanics and semi trailers loaded with parts driving around the country following the harvest. They will get you going again in a day or two, not 1-4 weeks later when some other dealer can fit you in or when the spare part you need arrives.
From JD's perspective that service network and parts stock costs a lot of money to maintain so it is available during harvest. They don't want customers going the cheap aftermarket route 11 months of the year because one month of sales can't sustain harvest support teams long-term. It's similar to peak electric generation capacity. Generator plants aren't economic if they only run 5% of the year at regular prices. In this analogy peak pricing has a limit: it would very quickly make harvesting the crop unprofitable.
FWIW I do think it's mostly a money grab by JD. Same reason Chamberlain closed their garage door opener API and has locked down myIQ: they want to force platforms, automakers, etc to pay for access to "their" customers and turn garage door openers into MRR. It's basically free money that you couldn't even imagine prior to the internet.