Evidentally

There are lot's of papers using the term "preliminary evidence." What is preliminary evidence? Doesn't that mean "evidence in preperation for evidence?" Does that make any sense? It's either evidence or it's not. Preliminary to what? Can something just be preliminary in general? Surely it has to be preliminary to something. What set me off was this person quoted  in Bright-Sided. The person says their paper provides preliminary evidence that happy people are healthier than less happy people. I assume she means the evidence is preliminary to future studies. But wouldn't all evidence be preliminary to future studies. This term has no meaning does it? I think I'm on to something. Or stupid. I can rarely tell the difference.