It’s for cases where a lot of content generated outside of the editor without needs to be able to edit it in the editor.
We see more and more tools that can export content into Defold format, for example Tiled, Blender scripts or TrenchBroom.
If after export you have ready-for-use content you don’t want to edit in the editor it will be efficient to mark such content as read-only. That will make the editor much more responsive especially in cases when you have a ton of content.
If you have more questions feel free to ask and tag @mats.gisselson , he knows more about it and can explain it better.