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Each of these is a separate service and there exist many domains that only use one or two of them.
For example, you can register a domain and not use it -- perhaps you want to protect it from competitors, or just save it for development a year or two in the future. In the early days of the web the domain name registrar was always a separate organization.
Many of my clients run several domains with web content, but only use email addresses on one of them. Another client uses a separate domain for email addresses because the domain is a shorter version of the long domain, whcih is their brand -- no html content on the short version.
So each is a separate service and there are situations where you really may want to use only one or some.