To add to what's said above regarding content downloaded from the DLS, the assets in the cache/internet folder will not have the same name of the asset you're downloading. You'll see something like 0knq899e205.cdp instead of an Ann Arbor 50 ft boxcar, for example. Content Manager will purge these files too after a while, and it's recommended you clean this out periodically if you download a lot of content because things can get screwy, and content may not install properly. Is it worth keeping these files? It's up to you but usually it's not worth it unless you can figure out what you've downloaded and also have the disk capacity to hold on to these temporary files.
For content you install yourself, we do more than highly recommend you keep the downloads you obtained from various websites. Those websites can go away, and your hard drive can die taking your installed content with it. Saving the downloads to the cloud or some other external device ensures you can reinstall them at any time.