I was also curious and found this youtube [1]:

Summary (found in video at 10:48)

1) separation of subcellular organelle (DNA/RNA) 2) extract solutions in biological fluids from aqueous to organic solvents 3) seperate lipid components 4) pelleting of ribosomes 5) pelleting of macromolecules

[1] https://www.youtube.com/watch?v=lRT_8nFc2Tk

Centrifuges are used for separation. An ultra is usually used for separating nominally-stable colloids (such as fine nanoparticle colloids), and miscible liquids where such high centrifugal force causes them to density separate.