Most of the affiliate portal approvals and/or certifications are peer-based reviews.
They check the site for original content (no copying), no spamming, no blackhat SEO, no selling daabases, no adult content etc. Generally having the “seal displayed” reflects that the webmaster is running the site in a good way, and behaving like a good internet citizen.
They may help with link exchanges with other sites, as most sites are usually more likely to do so with a site that has been approved by the same portal. They may help visitors to know that the website is not behaving in a “Rogue” manner.
They are not a requirement for affiliate programs.