A simple if statement allows you to run some code when your condition is true and do nothing if it is false. Rather than having to test again for the opposite condition we can add an else clause to the if statement to provide the code that is to be run when the condition is false.
A script that just makes one decision isn't going to be doing very much and so you can expect to have multiple if statements in your code to perform a variety of tests. Not all of these will necessarily be sequential in your code, in some situations you will want to nest one if inside another.
The switch statement also has a default clause that can be used to provide processing for all of the values the variable can have that haven't been separately listed. It also has a way to combine cases together easily where you want the same processing for multiple values.