To identify devices on a network as well as allow or block them on the network. Each MAC address is unique allowing networks to identify separate devices to grant IP addresses to allow them onto intranets or the Internet you are currently using. These IP addresses are then turned into named or fully qualified addresses such as the website you are visiting right now (ask.com). These MAC addresses are also used for communication purposes with various applications. For example, some database products use MAC addresses to bind a license to a specific host. This allows that host to use the license while other hosts may not use it. Further serving of the licenses to other hosts on the network can be done through communication to that bound license host.
Think about it kind of likened to a local phone number where you receive calls from people in your country. Your IP address would be how a foreign country would be able to contact your phone using a country code. Fully qualified addresses allow your name to be looked up easier via a phone book rather than just via your phone number.