Hard Drives
RAM sticks have a few issues in the memory department.
First, they can only hold around 8-64 gigabytes of info. Any more than that and they can get pretty slow.1
Second, when you turn off the computer they lose anything they are storing. This is a problem when you are writing an essay and need to turn off the computer half way through.
Hard drives are like RAM in that they remember stuff. The difference is that hard drives can store a lot more of it (terrabytes) and the data isn't lost when power is turned off.
1
The key word here is "latency". When the CPU needs info in RAM it needs to wait for RAM to find it. It takes longer to find something in a bigger pile of stuff, no matter how smart you are about it.