Why do you need to learn all the notes on the guitar neck? Because knowledge of where each guitar note is on an instrument is absolutely basic to any serious study of it. You must know where all the notes are to understand how to create chords and how guitar scales are formed.
For many guitar players the logic of the guitar neck is hard to visualize when playing lead. Fret markers solve this problem by removing any doubt about which notes you should be playing.
Guitar fretboard stickers stick to the guitar neck to guide you through the notes of the guitar neck and help you remember them.