Most piano students think of scales as boring drills they had to play over and over, but there is much more to them than just finger exercises. Scales are the “playing field” for songs based on a particular key. For example,
the F major scale is: F, G, A, Bb, C, D, E, F, while the D major scale is D, E, F#, G, A, B, C#, D.

Each scale contains primary chords for each key. For example, the primary chords (the most used chords) in the key of Eb are Eb, Ab, and Bb — the I, IV, and V chords of the Bb scale.

Every song is based on some key, and therefore some scale, which is why you need to know scales to figure out the most used notes and the most used chords in that particular key.

For more information, please go to:
playpiano.com/101-tips/Scales-Major-Relative-Minor-Special.htm