To be a better musician...you need to have better ears. Developing relative pitch (relative to the key, not as a leap from the note before) is the best way to understand how melody, playing tune and transcription works.
Relative Pitch in this topic is NOT the same as relative pitch as taught over the decades in classical studios. Any note, diatonic or not (in the primary key or not) has two main functions. It's either a tension note (a note that wants to move or resolve) or a resolve not (or sometimes refered to as a release note).
Knowing this AND hearing it unlocks the entire world of melody. Pretty much anyone can develop relative pitch and this topic tests you and gives you the basics on how to gain this great tool
Click on the image to view this 6 minute topic on building relative pitch