Regarding doing two or more songs in a row in the same key:

It's a subtle thing, but it's very real. Most people (other than very well-trained musicians or people with perfect pitch) won't even notice that the key is the same, but after hearing the same series of notes, albeit in different order, for several minutes, a subtle form of boredom can begin to creep in. You may not be able to identify the reason, but after someone sings several songs in a row in the same key, you may notice that there is something a bit dull about this particular singer, even if the songs themselves are very interesting and they are well sung. There's something about this singer that doesn't quite turn your crank.

There was a very good singer who was among us for several years. He had a really good voice, he knew all kinds of great songs—and he sang them all in the same key—G. Or Em, G's relative minor, which uses the same notes. I heard him a number of times in concert or doing sets at festivals. I enjoyed listening to him. But after a bit, everything started sounding alike.

I don't have absolute pitch, but I did notice that things tended to get a bit bland after a few songs. And I wasn't the only one.

Beethoven's Ninth Symphony starts in D minor. But it doesn't stay there. It modulates into other keys (as do all of his symphonies). The better, more experienced classical singers—and instrumentalists—are careful to vary the keys of the songs or pieces they perform in their concerts and recitals.

There is a reason for that.

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And another point to ponder:    Some night club performers, who do maybe four or five shows (sets) a night, will do the same show over and over again. This encourages turnover in the crowd.

Don Firth