Some of the concepts behind DVCS, such as the DAG, merging, resolving conflicts, even the staging area, are much easier to understand when you can actually visualise them. I've found that people who try to jump in at the deep end with the command line sometimes don't understand Git as well as they think they do.
These days I tend to use the command line a lot more, and I would strongly recommend people to do so once they've mastered the core concepts, but I still like to have a GUI to hand so I can see at a glance where things are at with my branches, staging area, etc.