Why are scales important for writing melodies?

[YoungCapone]

I’m trying to wrap my head around the concept of tonality and how scales fit into it. From what I understand, the relationships between notes (intervals) is at the core of how we write melodies and why they make us feel the way we do. It also seems that scales are built on top of those relationships and then chords are built on top of those scales. I still feel like I’m missing something there. Does anyone else have some insight into this? Wouldn't it be easier to just base everything off of the chromatic scale instead of having a million different scales and chords? Every interval relationship is already in the chromatic scale, why do we need others??

 

[Stereogram]

Diatonic scales have certain characters, which come from the different combinations of intervals (usually tones and semitones). It's because of the same interval over and over that the chromatic (and whole tone) scale sounds relatively characterless. All chromatic scales are equivalent (i.e. a chromatic scale beginning on G contains the exact same notes as a chromatic scale beginning on Eb, or any other note). Same goes for the whole tone scale, because there is only one interval. The same is not true for diatonic scales.

In a diatonic scale, the different intervals "point" to a root (I don't know the psychology as to why), but when every interval in a scale is the same it's like a flat, featureless terrain, which offers no "landmarks" to give a sense of where you are or where you're going.

To test the power of varying intervals, try going up a semitone when you should have gone up a whole-tone (or vice versa), it kind of throws you, putting a totally different "context" on the melody.

Another example, in Lydian mode, when you're ascending it sounds just like major until you get to the sharpened fourth. I don't know why but when you hit that fourth (going up a whole-tone), there's this incredibly "euphoric" sensation, when compared with the major (going up a semitone) - just from going up an extra semitone!

All the scales and modes have different properties like this, depending on their distribution of intervals, but I think it's hard to say exactly why.

 

[DRR]

If you write a melody without thinking about what key it is in (and therefore what notes ie scale you are using), then it is still in some key or other, except you've just found it by trial and error. By thinking about the key to start with you can save a lot of time. For example if I decide to write in Eb then I can look up the scale, see which notes are in the scale, and know that these notes will work as part of a melody. As Stereogram says, there's a number of variations of scales but you can usually stick with major or more likely minor harmonic or minor melodic.
You can also look up the chords that will work for pads and bass without doing it by trial and error.
Which key to use? Just google the most used keys in whatever kind of music you're producing. Then look up the scale notes and chords you can use.
Of course you can use absolutely any combination of notes, but they are quite likely to clash. The scales just work, and nobody really knows why the accepted scales sound fine to the human ear, and other combination of notes don't.