[RogerBW]

Well, I've written quite a few lyrics and several tunes, mostly for myself. But even though I took no particular trouble to make them performable by other people, other people have had no problem doing just that.

Writing for a specific performer might be respecting their limitations and/or showing them to best advantage ("I'm writing a show piece for Ivan Rebroff so I'll use all of the four and a half octaves that he can sing in") but I don't find myself convinced that there's enough that's distinctive about a performer that writing specifically for them would be a whole point's worth of knowledge.