Think of it in the same vein as going to your doctor, he draws blood and then runs tests to see what your body is doing.
same for an oil analysis, the results will tell what is happening inside your engine.
excessive diesel in the oil..... your injectors are leaking.
excessive metal in the oil ..... your bearings are getting hammered.
etc etc etc.
Its not rocket science and trust me, you <deleted> an engine and try and claim on warranty.....the first thing the dealership will do is have the oil tested looking for reasons to void a claim.
oil analysis benefits.