The CPSC helps these organization come up with the standards, and the organization charge a nominal fee for companies to be able to say they comply. In this case, it's seen as a badge of honor or something the company can show customers they are doing 'above and beyond'.
Another reason to join a voluntary standards organization is to actually help shape the standards, as these are usually consensus-led. Therefore, if companies want to have a say, they need to join and participate.
A complete of consumer products covered by voluntary standards is available here.
The Department of Defense (DoD) also uses non-government standards (another name for voluntary standards). Using these can help circumvent the long-winded, difficult process for updating regulations.
Their statement says this, "DoD policy encourages employees to participate as "equal partners" with private sector and other government employees on technical committees of the non-government standards (NGS) bodies that develop standards. Such participation ensures proper consideration of DoD requirements, enhances the technical knowledge of DoD personnel, and allows DoD employees to contribute their considerable technical capabilities to the development of "world class" national standards."
One method of encouragement is using these standards in their procurement practices. Contractors bidding would be encouraged "to propose commercial or non-Government standards and industry-wide practices that meet the intent of military specifications and standards."
Companies are encouraged to comply with (voluntary) standards like ANSI to remain competitive.
ANSI actually has several explanations for how voluntary standards are successfully compelled: "ANSI standards can also be interpreted as implicit regulations through our American legal system. The standards make a wonderful reference on how a machine should be, or more likely in the case of a trial, should have been, guarded. Employers or manufacturers who do not comply have a potential liability exposure if an ANSI standard indicates a method of machine design, operation or safeguarding that may have prevented an injury. It would be difficult to persuade a jury that a particular document is "just a voluntary standard" while the opposing lawyer advocates it is really the Holy Grail of safeguarding." and "It is also very common to see a purchaser of equipment, raw material, fasteners or just about any conceivable item reference that it must meet a particular ANSI standard for the buyer's acceptance process. "
Only the project owner can select the next research path.