Mostlikely there is a bigger enemy at play. Ohterwise the whole army mentality, weapons and goal for domination would just be pointless.
Well, Earth probebly wasn't the only planet with sapient life on it. Some of the colony my have had alien life almost as advanced as the Gems, so if Homeworld wants to conquer them their going to need all the soldiers and weapons they can get,
I agree with this. And also, to the original question, I imagine humans WOULD be a big threat. Afterall, look at our human history. If they didn't have a sufficient army, they would have trouble with how many humans there are. Plus humans have things like nukes and such. I imagine they also prepare for the worst. It's better to have too many than too few.
They had soldiers and weapons way before they even met humans otherwise how would they have started the Gem War in the first place. And I doubt humans are a big threat to Homeworld because they seem to merely be interested in building the geo weapon from Earth. If Earth is just materials for building a weapon than humans aren't the main threat. Plus after the Gem War, Earth became more or less a side project of some kind for Yellow Diamond as she wasn't as interested with it except for the geo weapon and it looks like the geo weapon is just one of many projects she's running.
That's not what she said, she said she learnt knighthood from humans.
I meant they're a threat in the sense that they'd fight back, if enough information slipped, anyway. If everyone found out, they'd of course attack. There are a lot of reasons the gems could have for what they did. It's probably just making sure they have enough to ensure their plans are successful.