I don't understand how and sometimes why Le or Les is used
Why is Le needed in the following Le doy una pelota a mi hermano
2 Answers
Actually, the le in that sentence is optional, but it is commonly used so I would recommend that you include it. There are other situations where it is required so it's simpler to use it than worry about if it is necessary or not.
Lesson 2.3 in the Learn section has a very clear 10 minute video explaining this. You might want to check it out.