Here's why you should never partially assemble a component with loose parts, with the intent to return later to finalize the installation.
Imagine that you are installing new hoses on a few seacocks. You carefully measure and cut several pieces of hose to length, and identify the proper hose clamps sizes. You want it all to look tidy and well planned, and toward that end, you decide to “dry fit” the hoses to make sure it all looks good. You slip the hose onto the seacock barb and leave the hose clamp loosely around the hose, with the intent to finalize the installation once you are satisfied with the hose runs and lengths.
Perhaps a phone call or a visitor interrupts your work, or maybe you reach the end of the day. A seacock with no hose attached stands a good chance of drawing attention, but one with the hose loosely in place will not.
Eventually, it will get your attention—when water pours into the boat.