Many people prefer to use a certain editor. They are used to the feature set and know the keybindings. OpenSCADs editor is functional and simple but might lack features people know from other editors. Moreover, external editor allows to use different monitors for editing and preview.
OpenSCAD is able to check for changes of files and automatically recompile if a file change occurs. To use this feature enable "Design->Automatic Reload and Preview"
Once the feature is activated, just load the scad file within OpenSCAD as usual ("File->Open.."). After that, open the scad file in your favorite editor too. Edit and work on the scad file within the external editor. Whenever the file is saved to disk (from within the external editor), OpenSCAD recognizes the file change and automatically recompiles accordingly.
The internal editor can be hidden by minimizing the frame with the mouse or by selecting "View->Hide editor".
In principle all editors can be used. Some even have extensions/modes to provide features for OpenSCAD.
openscad %directory/%filename
Besides using your editor of choice, these solutions enable the flexible usage of multi-monitor set-ups. One can have one monitor set up to depict the 3D object on the entire screen and a second monitor for the editor and other tools.