As already mentioned, you would want to use service hooks to help keep your team members updated on certain events that are happening in TFS when they aren't connected. For this type of integration, lots of teams can and do use a service such as slack to stay updated for when builds fail and new commits are made for source repos that they care about.

Another use is when you want to keep your community updated with the status of your project, whether it is updating a publicly visible Trello board or maybe something like integrating with User Voice.

The other occasion when you may want to use service hooks is if you want to integrate with another system that is part of your release and deployment pipeline, for example, using a service such as MyGet.