It is useful for some security protections in such a hosted subdomain scenario which is why herokuapp.com, herokussl.com, github.io, gitlab.io, etc are on the l

It is useful for some security protections in such a hosted subdomain scenario which is why herokuapp.com, herokussl.com, github.io, gitlab.io, etc are on the list, but yes, you would not be able to use your primary domain there, you'd need a dedicated domain for your customers and their subdomains. Namely, having your hosting domain on the public suffix list means that browsers will not allow setting cookies on the suffix (similar to how you cannot set a cookie for ".com"), which prevents rogue websites on the same root domain from stealing sensitive cookie values from other subdomains.