When you are writing on your Mac, you don't want to have to take your fingers off the keyboard to move the cursor or select text. There are a variety of keyboard commands you can learn to make it easy to move the cursor by letter, word, line or paragraph, delete characters or words, and make selections.
There are hundreds of keyboard shortcuts for Mac users. There are Mission Control shortcuts, Finder shortcuts, system shortcuts, application shortcuts and more. Many you can find by just looking through the menu bar for that application. You can find system ones by looking in the System Preferences under Keyboard.