The idea behind compressing/mergeing package files is to make your game load quicker. There is also some empirical evidence that having many, many package files in your mods folder is bad for game performance. (Which is why, as Crinrict states here, Sims3packs are actually better for your game than package files, if it wasn’t for all the issues that people have with the launcher.) The game doesn’t read the contents of the DCbackup folder when loading your game (it only reads the information that is written into the .dbc files found in the DCCache folder) so it would be a waste of time to compress its contents. An explanation of the purpose of the DCbackup folder can be found here. If you are not in the habit of exporting Sims or lots you can safely delete almost everything in your DCbackup folder if you are concerned about space, except the ccmerged file, which as you are probably aware contains necessary information about your premium content items.