Why write f flat instead of e?

Answer 1

Assuming there is an E flat in the key signature, a very common reason why a composer or songwriter would choose to write an F flat instead of an E natural is because you already have the E flatted in the key signature.

For example, lets take the key of Ab Major (four flats), in this case it would make more sense to write an F flat than alter the E flat so you don't have to constantly naturalise and flatten the E - which would make the written music full of unnecessary accidentals and difficult to read.

Even in C major (all naturals), for example if you wanted to add more expression when modulating to the dominant by raising the third and fourth degrees of the scale, you would use C, D, E♯, F♯ and G. You could write C, D, F, F♯ and G, which is okay, but it would look a lot neater if you used the E♯.

It is for this reason why double sharps and double flats may be used instead of the usual natural, and also why "white notes" having enharmonics come in very handy.