Unfortunately, many clients don't. In addition to their capacity to regulate members, professional organizations have the opportunity to educate the public, and be a valuable source of information to the media, government, public, and other pros since they're usually more accessible than individual shops. Having a certificate on the wall doesn't make your hand any steadier, or your artwork any better, or your aim truer, but at least it implies that thee client need not worry about catching hepatitis or another infection from your technique. Going to conventions is a great way to meet fellow artists and catch up on thee latest innovations. You just have to find it.
Most organizations boast quarterly newsletters to members with thee latest information.
When committees are being formed within governmental agencies, they usually look to members ov some kind ov organization. Probably because it instills in them the feeling ov legitimacy. There are many other great artists in the world who are not members ov these associations, but dropping by a group's website or office is a lot easier than sifting through thousands ov other options. At least these associations have a stance and a visible commitment to thee furthering ov thee art as safe. CABA, thee APP and APT aren't a bunch ov bureaurats sitting on high passing down commandments.