The system affects reputation, union membership, and income.
It affects reputation since some sources list only WGAE or WGAw certified credits. John Howard Lawson, the first president of the Screen Writers Guild (the WGAw's former name) said, "A writer's name is his most cherished possession. It is his creative personality, the symbol of the whole body of his ideas and experience."
The credit system affects eligibility for membership in the union, which is determined on points awarded based on what a writer has been credited for.
It can also affect income. While all writers are paid when they work, some contracts limit payments to writers if they are not officially credited. Additionally, only credited writers typically receive residual income from future exploitation of a film on video, pay-per-view, broadcast television, etc.
Read more about this topic: WGA Screenwriting Credit System