Since it used to be a very common prescription drug, they used it as an example of how our body gets rid of foreign substances.
As you can see, it first attaches a hydroxyl group, making the drug slightly more polar and thus more water-soluble. On that hydroxyl, it will further attach a glucuronic acid residue, which comes with 3 hydroxyls and 1 carboxyl group - making the metabolite polar as hell. I like to call glucuronic acid our body's "cement shoes" - a drug that normally isn't water-soluble (and thus also not "urine-soluble") can easily be eliminated via the kidneys after turning it into a glucuronate.