Many programs start out using "in-house tests" to determine English levels. It's a bad idea that can lead to problems from poor placement success to complaints filed with the EEOC.

I recently found and updated an FAQ developed by ETS some years back that is quite helpful in explaining in a brief format some of the reasons using TOEIC tests provides quality advantage.

Those who want a bit more detail can also explore this publication fully focused on explaining TOEIC reliability, validity, and fairness.