The Vercel platform provides the option to set up Custom Domains, with the ability to use Wildcard Domains. As described here, you must use the Nameservers method for verification for Wildcard Domains.
Nameservers method
This is because Vercel needs to be able to set DNS records in order to generate the wildcard certificates. The service that Vercel uses to generate the certificates requires us to verify the domain ownership by using the DNS-01 challenge method. By changing the nameservers, Vercel will handle the DNS-01 challenge for you automatically, and you don't need to update your verification DNS record upon your certificate renewal each time.
For non-wildcard domains, the verification is done using the HTTP-01 challenge method (which Vercel also automatically handles the challenge request for you), so you don't need to use this nameservers method.