Isoproponal or ethanol are used to precipitate either DNA or RNA during a nucleic acid extraction. Depending on the type of extraction the precipitated nucleic acids are then either centrifuged to form a pellet or centrifuged to be captured on a filter column.
If you are asking about the advantage of using ethanol over isopropanol or vice-versa, this is usually sample dependent. Here is a great blog post that discusses when you should use one over the other: