Why? Well, when I built my Meccano fretsaw it was primarily for fun, and then as an exhibition piece, and it seems to be a real crowd-puller to watch someone making a jigsaw puzzle on a self-built machine.
So I think if you built that beautiful wooden treadle fretsaw it would be a great addition to your stand at a craft fair. Of course the pieces you're selling would probably be done on the electric saw back home but what a great advertisement for your hobby!
Secondly, although you can perhaps take the principle too far sometimes, the idea of making your own tools is not a bad one. You learn how the tool works, for a start: what makes it good, what makes it bad. And there is a sense of pride in the self-sufficiency of it (although I guess until you can make your own blades too, that's a bit of a mirage...)
I can think of a third reason, just about: to build a saw with some specific characteristics to enable you to do something you couldn't do with a commercial saw. One might be speed control, to have the blade moving really slowly like with a Diamond saw that can go as slow as you like. Another might be to have an exceptionally large throat for those occasions where you might want to cut a very large board - with a jigsaw puzzle, for example, you'd only want to make the one (very long and wiggly) cut down the middle of a board to double the size of jigsaw you could cut on your electric saw.