That's a terrible analogy. You aren't buying supplies, you're building something. Why not build it as strong as reasonably possible?

A better analogy would be like building an upstairs room but only using 1/3 the floor struts. So it could only support a certain weight. It can support a bed and dresser and that's all you need so it's OK. Then you want to get a desk, but it can't support all 3 at once. You can get a desk but you need to get rid of the dresser. Or you could do what you're doing here and argue "you don't need a desk, you can get by just fine with a dresser and bed. There's no reason to use extra floor struts."

Or I can build the upstairs once and use shit tons of floor struts and not have to worry about weight. Then I could have all sorts of shit in there and not worry about the weight limit.