Different types of memory make computers faster. For example, disk drives are *very* slow -- so manufacturers put a "cache" in the drive, using statistical methods, software predicts what parts of the disk will be accessed next, and pre-loads them into cache, so that the next time you make a request from the disk, there's a chance the data has *already* been retrieved. The faster the memory is, the more expensive it is -- so you need a hard drive to store operating system and everything else, and you need cache to make that faster, then you need main memory, that's where disk files go after they're retrieved, so the computer can process them, that has cache memory too, to make it even faster.

If I understand your question correctly, you're confused about the functions of various pieces of computer architecture. System memory, or RAM, is what the computer uses to hold programs it is running and data that it needs quick access to. The RAM's contents vanish when the computer is turned off. The hard drive, or other storage devices, such as USB flash memory, CD-ROM, or floppy disks, is used for long-term storage. The data stored in these devices remains there even if there is no power. Cache memory is a small amount of RAM attached to the computer's CPU (central processing unit). This memory is similar in function to system RAM, but it is much smaller and the computer can access it more quickly. There is also something called swap memory, which is where the computer supplements the system RAM by using part of the hard disk as if it were RAM. This is much slower than using the RAM, both because the data has to traverse a cable and because it requires the hard disk's mechanical parts to move. A computer can function without a hard disk, though it makes it far less useful to not have storage. It can operate without cache memory quite successfully--all you'll experience is slower performance. Theoretically it can operate without system RAM, but that hasn't really been done since the days of punch cards.

You wouldn't want only one type of memory because of the specialized function each type provides in a computer. Imagine this analogy - you are the CPU. Your brain is the cache. Your desk is RAM. Your filing cabinet is the hard drive. If you are running a program, you get it from the filing cabinet (which holds the data even when there's no power), put it on the desk to prepare to work on its various parts, your brain looks at the work and can remember small parts of what it sees at any one time (but not the whole desk's contents) and you process what is in your brain. If this analogy makes any sense to you, you'll understand why each memory type is important. Without the brain (cache), you'd have to keep looking on the desk over and over (RAM) to remember what it is you're supposed to process. Without the desk (RAM), you'd have no place to hold the data waiting to be processed. Without the filing cabinet (hard drive), all the data would be gone if the power was shut off. I use this same analogy to describe virtual memory (swap file). If your desk isn't big enough (not enough RAM), it can only hold so much work. When more work needs to be pulled out of the filing cabinet, something on the desk has to be put away for a while - so it gets put back in the cabinet in a special drawer that can be used just for stuff that was being worked on the desk.