Question: 

Is the main function of USPAP to protect appraisers?

Response:

No. The first paragraph of the PREAMBLE states:

The purpose of the Uniform Standards of Professional Appraisal Practice (USPAP) is to promote and maintain a high level of public trust in appraisal practice by establishing requirements for appraisers. (Bold added for emphasis)

Although the main purpose is not to protect appraisers, appraisers do receive significant benefits and protection from USPAP. It establishes requirements for impartiality, independence, objectivity, and competent performance. Because of these standards, appraisers who comply with USPAP are viewed as unbiased professionals whose work is worthy of public trust.