Computers use different kinds of storage.
This is because storage media is either slow and cheap or fast and expensive. The L1 cache transfers data at several hundred million bytes per second. This is computer speak for describing the fact that you tend to use the same small amount of stuff repeatedly, and most of the other stuff much less often. It's like the fact that you tend to drive around town a lot more than you drive around the rest of the country. When your computer goes to reference a location in the many kinds of storage, it first looks close by to see if it used that location recently.