Forks are needed when the network requires of major or technical upgrades, the ones which changes the rules of the protocol. The fork can also be used to:

• Fix bugs
• Make changes
• Changes of the governance structure of the DAO

It's like a software fork, where developers create a new version of the software, but in this case applied on a decentralized network.

Once the Fork is made clients must update their software to implement the new fork rules