items that exist ambiently in the world are equally capable of existing ambiently in your ship hallways and bathrooms. There are a number of consequences to this. Only cargo properly stored in the cargo placement area will show up on your manifest log and be eligible for selection at kiosks and trade stations - otherwise, you'll have to cart that big benny's machine all the way into the store before you can sell it.
Additionally, because it isn't on your manifest, you can't manipulate it via standard cargo management features:
Loosely placed items aboard the ship are vulnerable to damage from impact and maneuvers.
Loosely placed items aboard the ship are liable to damage the ship during impact and maneuvers.
Loosely placed items aboard the ship are liable to cause serious harm to individuals cohabitating with said items during impact and maneuvers.
The cargo area allows the server to easily save the state of things aboard the ship when parked at stations or hangars - depending on performance impact, loose items may be vulnerable to getting lost in storage.
The cargo area also handles many of the properties governing concealment and scanning, which means that loose items may not get the benefit of that sweet smuggler bay. Essentially, the intent is to allow things to be placed around the ship to retain consistent behavior throughout the game, but actually doing so is inefficient and risks damage, discovery, or death.
Tampering with cargo area can something something void your warranty, rules and restrictions may apply. Consult your doctor before use, and remember to fly safe.