This is a good question as hiring the right person is always an important challenge.
The benefits of using tests are as follows. First, since the test is a standard test, all people who apply will be taking the test. This limits favoritism. Second, a test does measure specific things and may be a decent indicator of a person's potential. For this reason, you will get a sense of what the person is like.
With this stated, there are also downsides in using a test. First, many tests are poorly constructed. Therefore, they are not great at measuring intelligence. Moreover, some intelligent people are not good test-takers. A great person may fall through the cracks. Finally, the opposite is also true. Some people who are less than intelligent and honest might score extremely well on a test. In this case you will be hiring the wrong person.