I'm not sure why either! I guess it might be useful in case conceptualization. Maybe if therapists give a certain test enough times, they can even tell how their patients might score.

There are nine scales for things like hypochondria, depression, paranoia, obsessiveness, etc and then a bunch of other things. I wasn't really clear on whether these measure those things as personality traits or symptoms- I think that part might be open to interpretation though since the test doesn't really say, "how do you feel today," or "how do you feel usually."