So-called "classical" or "art" songs are often published in editions for low, medium or high voices. Books of pop songs, songs from musicals, cabaret songs, chansons, etc., are usually in keys without too many sharps and flats. I sometimes wonder why they're in one key or another. It's always a compromise and anyone who wants to specialize in accompanying singers will have to learn how to transpose at sight.

For example, I have a couple of books of songs composed by Friedrich Hollaender. I know that some of them were written for good female singers with high voices. Many of them go up to a high G --- not reachable for many singers and certainly not for me.

Quite a few songs by Noel Coward are published in Eb maj. In the introduction, he writes that this is his favorite key to play in. I don't know whether there's a connection.

Usually, songs will need to be transposed because of the high notes, but sometimes one will run across a song which goes too low for one's range. Some songs just have a range that's too large.

Another problem is songs that stay in the upper part of one's range for too long. John Dowland's songs are like this for me. I can reach a high D, but I can't stay up in that region for too long. While one can easily use a capo to transpose upwards, it doesn't work in the other direction,