Adhoc testing is used to <b>catch the hidden bugs</b> in the product.
It has been experienced in testing that functional test cases derived out of product requirements stop yielding defects after few builds or test cycles.
The tester therefore tests the product relying on <b>skill and intuition and on his experience </b> with similar products. He travels on <b>different tracks</b>/ use-case scenarios than what has been specified or could <b>vary the test case</b> that results in a defect to find out more cases where failures can happen.
So, Adhoc testing can be used to determine the ability of the tester to some extent.