Can anybody give me some information about dat machines? I understand you hook up the mic to the dat machine, record the sound seperately from the video, and then with editing synch it together? Is this essential for creating good sound?

Your description is correct, except DAT recorders are aging out. They are being replaced by recorders that use hard drives and solid-state memory cards.
In addition, camera audio has been improving lately. Depending on the camera, and audio equipment hooked to it, good sound is possible without using an external recorder.
If you want to be physically separate from the camera, but don't want to depend on wireless transmissions to do this, then an external recorder must be used.