I manage a multi-tenant SaaS, and that use-case has been mentioned. However, neither of those threads mentioned specifically that a wildcard certificate is necessary in order to conceal valid subdomains. In this case it's not just a matter of convenience, but of security.
As an example, if "Company 5" signs up for a service at example.com, they may receive an address of https://company5.example.com. When they visit, they're required to login. If someone visits an invalid subdomain (e.g. https://company7.example.com), the connection fails, telling the user that this is not a valid subdomain.
In this way, a malicious user is able to deduce which subdomains are valid accounts and which are not. With a wildcard, we're able to present the same login dialog even for invalid subdomains, albeit they would never be able to successfully login.
Thanks for reading.