For me it is a multitude of reasons: Preserving history, sentimental & function.
I believe we need to save what we can of the past. These machines represent a period of history that we will never be like again. Some of these machines come with a story (great-grandma's machine someones mom learned to sew on & passed down) or they have a story that will never be known.
I hang onto my 2 vintage machines for sentimental reasons. I received my 1947 Featherweight as a Christmas gift from Mom & Dad when I was 12 years old. Which means I have owned it 39 of its 64 years of existence! My 1956 Singer 403 was my late Mother's. I used it high school to sew all of my 4-H projects. Now that I have it at my home, it is my main machine to piece my quilts on.
I love the way both of my machines sew. They are tough, they run great & for their age - have less breakdowns that I do!
I will use both of them until they just won't go anymore. Then, and only then, will I buy "new". Well, unless I find another vintage machine that is...:)