In answer to Richard, who said:
"all the white notes is C major or A minor scale. It can't matter which order they come in"
- Then how do you know whether it's C major or A minor? ;-)
Answer that, and you'll have an idea of why modes matter (or, OTOH, don't).
IOW, what about a tune that uses all the white notes, but starts and ends on Dm? Is it "in C major", or "in A minor"? Or is it "in D minor"?
Or is it - as us overeducated folk might say - "in D dorian mode"? ;-)
(NB: you don't have to care. It's only words...:-))