I play solo often. It helps in learning rules. It helps to develop a sense of possible strategies and gain some mastery of mechanics. Sometimes I play out limited scenarios in “what-if” mode. Yes, if I develop a strategy, my “opponent” is fully aware of what I’m attempting. When I switch sides, I look for the best possible response and thus can learn something about how my strategy can be countered. I have sometimes played out entire games like that, because it’s fun to see how different choices (and luck) work out. If a strategy turns out to be fatally flawed and disaster ensues, that becomes part of the narrative of the game. I’ve also made maps for small scale engagements, inspired by a wonderful game that imagined a British invasion of Heligoland-Bight (I am sorry I can’t recall the name). My current interest is a map of Ceylon imagining an invasion to take it as a stage for invasion of India.
I can also whimsical stuff just for the hell of it. Alternate histories and just weird stuff. A recent setup of 1942 became The Great Orc War when genetic mutants called Orcs because Orcs came swarming out of Mongolia in mid-1943 (dictated by die roll). Determining things by die roll can be a way of limiting bias. Die rolls determined when the Orcs appeared, how many of each type appeared in a turn, when they acquired certain capabilities and what those capabilities were, how nations responded to events, and so on. In addition to die rolls, it is helpful (as already suggested) to define objectives and any unique tendencies for each power. Sometimes those become principles that can make a decision, and sometimes they can be reasons to tilt the die roll. The response of the Japanese in the game was assigned to the “very unlikely” pile but the die just happened to go there.
In the unlikely event that anyone would be interested, the Orcs were defeated, the Soviet Union was destroyed, the Japanese were contained and cooperative, the UK was a shell of itself with colonial possessions lost, the Germans were defeated by the US and UK despite an alliance with the Orcs, and the US emerged as the only surviving great power with some interesting tech developments. This matters because I’m thinking of some new events in this alternate universe. :)