Plesk allows you to subscribe to as many hosting plans as you need. For example, if you do not want your main site to share hosting resources with a company’s branch site you can subscribe to another hosting plan under the same customer account. In other words, a customer account is a personalized access to all your subscriptions.
Another reason for using multiple subscriptions is that subscription is limited to only one IPv4 and one IPv6 address. Thus, all domains within a subscription share the same IP address (v4 or v6). If you need to host sites on different IP addresses, you should additionally order the same plan for as many IP addresses as you need. This can be useful, for example, if you want to secure connections to each of your sites with a separate SSL/TLS certificate.