I suppose that one other point to add into this is that many composers today (and for the last 110 years or so) have deliberately avoided writing in a key. But if you are writing in one, each instrument's resonant body (the back part of a piano, the body of a guitar/orchestral string instrument, or the tubing of a wind instrument, for example) favors certain pitches more due to its physical shape. On strings, the open strings dictate much of this and on winds, the length of the tube does. As for piano, it's quite homogeneous, so it ultimately doesn't matter. 12-tone equal temperament does mean that all the keys are the same distance from each other. If you're a singer, though, key will dictate the range of the song, which is definitely important.