To tell you what is going on inside that engine. Also great to use as an indicator of when you really should be changing your oil. The test reveals the contents of the oil which are very useful in determining this. For instance, if the test shows water is in the oil then you may have a cracked head or a headgasket that leaks. Also, several chemical tests reveal the condition of many aspects of the oil which determine how close to "new" it is - basically, if it is still usable as a good engine lubricant or not. This is what many companies with fleets use to determine their oil change intervals. I've even used it myself. I was skeptical of Amsoi's claim that its synthetic oils are good for 25,000 miles between changes until an oil analysis of the oil in my [now sold] Toyota Avalon revealed that even after not changing the oil for 25,000 miles, it was still good for continued use!!
Truly amazing.
You can purchase an oil analysis kit from Amsoil here:
https://www.amsoil.com/storefront/SearchResult.aspx?CategoryID=98