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!