How should a composer use Scales/modes as a tool for composition?

1. How do composers use scales to make music?
2. Why do composers use different scales?

How do composers use scales to make music?

So scales can used write melodies, harmonies and can be mixed with one another, alter the functionality of a chord (it may change the 6chord from tonic to sub dom function for example).

Why do composers use different scales?

This may be because most people begin their musical training at the keyboard with pieces in C major, and with this key as the norm automatically "feel" sharper or flatter keys in relation to it. Certain keys have come to have certain associations: C Major is open and noble, E flat major solemn and imperial.