This is a safety net so that assets aren't overwritten by accident. Sometimes, assets can have the same kuid number. This is common when assets are exported to cdp files and then removed from Trainz. At some point, there's a possibility that the same kuid can end up being used again at some point because Trainz no longer has any reference to the one that was deleted. What you don't want to do, for example, is to import a building you have created that has the same kuid as a route you've been working on for ages and end up losing the route! To get around that, Content Manager is configured so that if the SAME kuid exists, the asset is left open so you can check first and use your thinking cap and brain to think before blindly overwriting your already installed assets.
What I do when installing cdp files is open up the view in Content Manager with Open for Edit showing instead of Installed Content. I then drag in my cdp files and once everything is installed, I then revert all the open assets to original unless the content creator says specifically that there's an update for something. I got into this habit years ago and it has worked perfectly.
@pware
Sometimes there are asset dependency packages for various locomotives, for instance that use the same components or are included in various downloads. A good example is some of the diesel locomotives where the same components such as sand, diesel fuel, cab interior, and bogies are commonly used across multiple models. Some authors will create a dependency pack while others will put all the assets in a single pack. While that's convenient in some ways, it can cause the open for edit issue if there are multiple models of the same locomotive since each pack that's installed contains the same dependencies.