I often used it, and saw it used, in my field of histopathology (diagnostic study of human tissues). A typical phrase would have been "admixture of blood clot and tissue fragments".
To me, it helped to convey the fact or observation that two or more elements were present together, but still separate. So, in the above example, it meant you could clearly see that there were bits of blood clot and bits of tissue, rather than a mass of bloody tissue with the two elements being indistinguishable from each other.
However, I see that the OED defines it (as a verb): "To mingle with something else; to add as an ingredient" (my italics).
Whereas MIX is defined as: "To put together (two or more substances, or groups or classes of things) so that the particles or members of each are more or less evenly diffused among those of the rest"
So that would allow us to say something like, "Prepare a dough of flour and water and then admix the raisins." Just saying "add" the raisins doesn't tell you to add them and then mix them together with the existing ingredients.
But with the word "mix" you would have to say "mix in" the raisins. Admittedly, being able to leave out that "in" doesn't exactly make life a lot easier! But hey... maybe in some situations I could imagine it being handy.