A good baseline run test can be a great way to retest your fitness every couple months to make sure you are improving. There are also a few other benefits that go without saying. Especially for the all out 5k test.
You can look at and improve: Biomechanics (form)- The faster we run the less wasted movement there is. Running faster sometimes helps us identify those inefficiencies. It's fairly accurate in predicting longer efforts. Use this calculator to find out what your 5k time predict for other distances up to the marathon. It's an aerobic test. Important for the type of running ultra runners are doing. It's easy to retest. Just logging the time it took and your HR gives you the data you need. Mental toughness. It ain't fun for most, but it'll make you tougher. That is measurable too!