Why/when would you use a light to frequency converter?
The data sheet for the TSL235 points out 2 important advantages of the device:  1. The output is in digital form without the use of external components, and 2. the device can be interfaced directly to a microcontroller.  It is temperature compensated, has excellent linearity, and the package includes an integral lens for the light input. Combining all these features with others given in the data sheet makes the price very competitive with trying to achieve the same results with individual components.