The servo testers which can be found in shops are quite expensive and the shematics published here and there do not achieve the requested precision. Most of the time they are based on the famous NE555 timer IC and are limited to one function.
The use of a PIC (here 16F84 or 16F628) gives the opportunity to build a very cheap and extended tester. Such a project costs from 10 to 30€.
With this realization, you can test a RC servo but also check its correct implementation in a RC model without having to fully deploy your RC transmitter/ receiver. This is quite more convenient. After all, we have only two hands….