The easy explanation is that a complaint written by an attorney not only states what the discriminatory act was, but shows how this act meets all the requirements of the law to be illegal discrimination.
For those people who wish to proceed without attorneys, they may meet with an intake person or just fill out forms on their own to be presented to the state or federal agency. The complaint will be taken from the information obtained on the forms. On the other hand, for those with legal representation, the attorney will meet with the client and ask questions about the discrimination, what happened, when it started, who did what, whether the employee reported the incident, and other relevant information. Using current employment laws, the attorney will draft a complaint that shows how the client was discriminated against and how the instances complained of meet every aspect of the law. The attorney will then file the complaint with the state or federal agency. Once an investigator is assigned, the attorney will work with that investigator to fill in missing information, provide evidence, answer questions, and propose a settlement. The attorney may also contact the employer prior to filing the complaint to see if a settlement can be reached.
Also, the fact that a person has gone to the expense of hiring a professional to write the complaint shows that the employee feels that his or her complaint is so serious that he or she is willing to put money on it.