A typical use case. User is going through their RSS feeds. As they read each article, they click links within those articles. This results in a few trees under the RSS feeds parent tab as shown in this picture.
User is done with RSS feeds for now. They would like to close all the trees that spawned under the RSS feeds tab without closing the RSS feeds tab itself so they can resume reading news later. An option in the right click menu along the lines of "close all children" would help with this.
For what it's worth, Firefox's tree style tabs extension supports this feature.
There are valid scenarios where users want to close entire trees but there are also valid scenarios like described above where multiple trees spawn under a main persistent tab. Feedly, Reddit are good examples.
Adding ability to close all child tabs will let users freely open multiple tabs under websites like Feedly, Reddit and then close all the child tabs at once while retaining the main tabs which are probably persistent tabs that are always open for most users.
