It give you some important information about your computer and what it currently is doing. Probably the most important tidbit of info that is given is the amount of memory the application is set to load with. In order to accommodate a very large text file, you may have to increase the amount of memory devoted to that application. Or, if you are running low on memory, you may need to decrease the amount of memory an application uses in order to get it to run. You probably won't have to worry about this much anymore, since Macs come with more and more memory, and more importantly, virtual memory is becoming standard.
Disks are much slower than memory, but they tend to have a very large storage capacity.
When you're done using the Mac, you should shut it down, but just turning it off is NOT a good idea. The Mac, depending on the model, will either turn itself off, or do some shutting down operations and tell you when it is safe to turn off. You should always use the Shut Down menu item, in the Finder's Special menu. This will allow the Mac to prepare for shutting down, by closing all open applications and closing any files that remain open. Turning the machine off without choosing the Shut Down menu item may cause loss of data and a corruption of files.