Welcome to the forum, NoreePooh!
I want you to know that I removed the color from your post as per forum standard. We ask that everyone use standard black for their posts to help keep everything neat and tidy.
As for your question, if you delete the items from the download tabs, you're just deleting the originals. The actual files can be found in the downloads folder within your The Sims 3 folder. I tend to move those files to another location to save the originals in case I ever do need to reinstall for whatever reason. This in no way effects what has already been installed as it's just the files you need to install the item not the actual item from your game. Keeping them there doesn't really effect the game in anyway, just your launcher and how long it takes to load it up. Plus as you said, not having anything in there makes it difficult when you need to install something new. What makes the game take a while to load is the actual amount of items installed.
I delete my DCBackup files and the other cache files (except socialcache and ccmerged) every time I load up the game. This seems to help my game run better and any glitches that I experience can sometimes be taken care of by quickly doing this and then reloading the program. If anything, your game will just run better.
