Army Camp
This building is full of cars because it's an army camp. The cars (and tanks and planes) need to be ready to go at any moment just like the soldiers.
Picture an army camp housing all of the different cars the army needs at any given time.
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All of the people in the army camp are wearing jeans (じん). You thought soldiers were supposed to wear uniforms, but it looks like when they're in the camp they have to wear jeans.
Jeans are pretty durable, now that you think about it. And if the soldiers are working on these cars in the camp while they're there, it makes sense to wear a nice pair of jeans.