It isn't required, honestly. I am using a number of RichTextLabel nodes in my current project without issue.
If you have your node inside a special 'organization' container such as an HBox, VBox, ScrollContainer, or the like then often times the node will be squished down to nothing as it attempts to allocate space for the container. It depends on the child as to how this happens. Setting a min_size won't allow the container to squish it past that point thus addressing your problem.
Something I do often is just checking the "Expand" boxes under the "Size Flags" of the node to make it take up the remaining space in the container. This way I don't have to have some exact size in mind.
I do agree there should be something, even just a warning, in the manual about the containers auto-adjusting margins and the like for their children. While it may seem obvious to someone used to Godot it did drive me up a wall at the start as well.
