There are, essentially, 2 methods of recording sound for a motion picture:
1. Single System, in which the sound and picture are recorded simultaneously on the same medium.
2. Double System, in which the sound and picture are recorded on separate media and synchronized in post.
If you're just shooting a small project on video with minimal crew, recording double system is unnecessarily complicated. As you say, just plug in an external mic and go for it.
Double system is used on pro shoots so the camera can operate relatively unencumbered, and so the camera op and sound recordist aren't in each other's way.