The weight of the material is supported by your shoulders, and not your hips, which is far more comfortable in general
In addition to having more room for pockets, overalls can carry heavier things without falling down, nor need for a belt
Since the weight of the material is carried at the shoulders, bending down at the waist is easier and less restricted. The waistband no longer needs to be tight, since it no longer serves a functional purpose
Well OP did specify the garment was supposed to be loose fitting. I’ve seen sausages with looser fitting skins than TNG uniforms.