If enough people make them resort to bloodshed instead of going quietly, oppressing the people becomes increasingly expensive and logistically difficult. Their tyranny thrives on acquiescence.

Guns (probably) aren't going to stop them if they're intent on destroying you but if they have an agenda other than bloodshed for its own sake, making the path of tyranny as costly as possible can make all the difference. Enough people banding together could deter them entirely, or at least force them to regroup.

The true effectiveness of civilian deterrence against a modern government might not be known but logistics, expenses and optics make a difference in any conflict.

I can't say how committed the "fuck around and find out" types are but I know it's always better to have guns and the will to use them than to have neither