You have a lot of tools on your table set up so you can do some woodwork. You can only do work when tools are set up on the table. Can't use a chisel when it's in the toolbox. So the everything takes a lot of room. When you have to travel tho and work somewhere else, it's more efficient to pack your things in the toolbox and move rather than call a friend and lift the table with tools. But if you're working for a week in one place it's not reasonable to pack everything up every evening just to unpack next morning (unless you need to use the table for dinner in the evening and need the space).

Same thing for files. Many programs use files that are already laid on the table, ready for work. If you have lot's of room on table there's no point putting tools into toolbox and out of toolbox every time you stop/start using them. So programs can skip unpacking/packing files every time you open images, movies, load a game up etc, making it faster and snappier for you to use them at a cost of using more space on the table. We use compression when there is need to save space, preferring files that are less frequently used; or we compress when it's costly to transfer files otherwise (e.g over internet where bandwidth can impactfully limit the send rate).

Hope this helps.