The velocity of the object is similar to speed but has one main difference: velocity takes into account the direction of an object. Since speed does not take direction into consideration, it is considered to be a scalar quantity. Thus, the speed of an object can never be negative.
My answer: Because the actual thing you might be modelling might be too small or large to replicate, (for example you cannot draw a plant cell TO SCALE because it’s way too small to see with a bare eye).
And if the item is too big, you won’t be able to fit it in a small space; if you were to draw it or create it in 3D.
I hope this helps!
Sorry if the response is too short, try rewording if you need to :)
Oxygen , Carbon , hydrogen