Benefits of being a guest editor of a journal?
Disciplines are typically organized in loose groups around a fairly small number of persons. They organize conferences, are editors, are on boards of professional organizations, etc. They are always looking for the next generation, new researchers who could take over when they retire. Being a guest editor for a journal, organize a workshop, chair a session, etc. signals that you might be the next generation. Having a reputation of being the next generation won't hurt when you aply for your next job. It is also good practice for when you become a regular editor.