Each domain name which is registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be set up by hand if necessary. For example, the A record of a domain name is its IP address i.e. this record shows where the website is located; the MX records show where the e-mails are managed, while using an SRV record you can use a domain for unconventional purposes, like a Voice-over-IP service. To be able to set up and modify specific records allows you to have different services with different providers if, for instance, your current host provides stable web hosting, but has troubles with the email service, or you simply prefer to get a particular service from a specific company. Although you will be able to set up and modify these records freely and with no limits as you own a given domain, some companies charge extra to give you this option.