Scientist most likely study Embryos due to the fact that they are barely starting to develop.
We originally study an Embryo to see how humans change and form. So while Embryos are growing and developing into babies. We can study them and see how we change as a whole.
And in Evolutionary Relationship, we do it to help see how animals evolve as well.
Remember that all of this has something to do with Evolution and the change of Physical Structure.
