Biologists use it to, among other things, predict the time that vaccines will stop being effective, prescribe antibiotic regimens to prevent the rise of antibiotic-resistant infections like MRSA (and predict and explain their existence in the first place), and explain anatomical and physiological quirks that would make no sense otherwise (like allergies, appendicitis, etc).
You will find that, on close examination, there is not a solitary example of applied biology that is dependent on the theory that all life on earth evolved from unicellular organisms. The cult of Darwinism wants people to believe otherwise, of course, which is all part of the deception. From this deception comes nonsense like “the theory of evolution is the unifying concept of biology” and Dobzhansky’s (in)famous claim that “Nothing in biology makes sense except in the light of evolution.”
The truth is, biology doesn’t need stories about whales evolving from a deer or humans evolving from a monkey-man or chickens being dinosaurs-in-disguise. What is useful to biology is the facts of here and now, not bizarre tales and curiosities of ancient history.
And be aware that under the wide umbrella of “evolution” are found both facts and non-facts. For example, antibiotic resistance is “evolution” and is a fact. On the other hand, while the theory that humans descended from a monkey-man is also called “evolution”, it is certainly not a fact.
So if you google “medical applications of evolutionary principles”, you will find many medical applications for certain factsthat are found under the umbrella of “evolution”, but you won’t find any medical applications for the non-facts that are found under the umbrella of “evolution”.
The moral of the story is, don’t be misled into thinking that, since it is a fact there are uses for “evolution” in applied science, it must also be a fact that all life on earth evolved from unicellular life forms.