Internal DNS comes in handy to solve internal server resolution. For instance, a mail server can have a DNS like mail.company. This DNS record will work for all clients connected to the realm on which the DNS is added. DNS resolution works for all applications running on client computers, from a simple ping request to a browser like Chrome or Microsoft RDP client.
Upon addition of entries, client records will update immediately using the Update clients button.
Apart from the explicitly declared DNS records, all computers in your realm have an implicit address which is computer.realm. Say your realm is contoso and a computer in your network is John_Doe, this computer will be reachable under the address John_Doe.contoso.