You say please when you are politely asking or inviting someone to do something.

You say please when you are accepting something politely.

You can say please to indicate that you want someone to stop doing something or stop speaking. You would say this if, for example, what they are doing or saying makes you angry or upset.

You can say please in order to attract someone's attention politely.

You use please in expressions such as as she pleases, whatever you please, and anything he pleases to indicate that someone can do or have whatever they want.

If someone or something pleases you, they make you feel happy and satisfied.