Another reason for choosing a key with more flats is that it sounds "darker". Round Midnight for example is written in Eb minor. It doesn't get any darker than that.D.Cass wrote: ↑Thu Nov 02, 2017 6:25 pmIf a session player is asked to play in Eb, they would simply play in Eb. No need to to retuning the instrument. One may find themselves in Eb in a blues seeion if there are horn players, so it is not all that uncommon. The only time needed to retune would be if open strings are required. Maybe some would use a capo, but bar chords work just as well.
If you are asking why do some rock guitarist tune down to Eb. Well, there are several reasons. One could be the vocal range. Having played with vocalist I have found myself in Eb often. “Hotel California” was orginally written in E Minor, but it didn’t fit the vocalist range very well. Another, is a heavier sound out of the instrument. There are probably more reasons for dropping down Eb
I have to admit that my own arrangement of Round Midnight is also in E minor. However, one could use a capo on the 1st fret which would transpose it to F minor with 4 flats for "darkness".
In a rock context definitely tune down a semitone. This is not actually a choice - it is your duty to tune down - since the typical reason rock/metal would play something in Eb or lower drop-tuning is to get the extra low frequency colour from the lowered-by-semitone sound. Some, like Metallica on practically the whole "St.Anger" album, would tune down to C (!). https://en.wikipedia.org/wiki/Drop_C_tuning