There are a couple of reasons for an external mic jack...

1) Lets say you are at your child's piano recital. You - the camera person - are at the back of the room... a solid, hard wall, hard floor and possibly hard ceiling environment where every spoken word and note will echo off of each hard surface before and after it gets to you at the back of the room. Since the mic is in the camera, it hears what you are hearing... and then some.

If you had a mic up on the piano, the audio you would pick up would be far superior to that which you hear at the back of the room. If you don't have an external mic jack, you would never have the opportunity to even try to grab that audio - unless you were allowed to place the camera on stage, next to the piano.

In this case, a stereo mic would be advantageous to be able to hear the stereo separation the spatial differences that go with the distances between the high and low notes.

2) Let's capture a family moment from a distance - some people don't do well with a camera in their face... If you are 20+ feet way and want to pick up the conversation along with the action, the camera-mics may not be able to hear that far away. A wireless mic clipped to one (or more) or the subjects will bring the conversation in to the video.

3) Some camera mics are not mounted in a way that isolates them from the camera - consequently, it is possible that the camera will pick up its own noise - a tape motor or hard drive motor (or a DVD motor). This typically happens when the auto-volume level control is in automatic (default) and there is not much ambient noise. This can get very irritating.

If all you are using the camera for is to take video and all that video will be stripped of its audio because you will add a separate audio track, then the external mic jack is unnecessary - but this is not typical of most users... they generally grab audio and video together ... and while there might be some music added, it is generally not used to replace ALL dialog.

Please keep in mind that the Panasonic PV-GS series will be recording onto MiniDV tape. This is a digital format and generally will not have the issue of hiss associated with older analog camcorders.

Microphones, as with your ears, are very sensitive. Camcorder mics are not exactly the most sensitive mics available - but to think that they are the best solutions for capturing audio with your camcorder would be a bit of a stretch. You would normally add music to your camcorder-captured video and audio when you are editing on your computer...

Which takes me to the next point - which you did not have in your question... In order to get the video and audio from the camcorder to your computer, you will use the camcorder's DV port. This is a 4-pin FireWire connection... the other side of this cable will be either a 4-pin or 6-pin connector depending on the type of FireWire port you have on your computer. Please do not confuse USB with FireWire - they are very different. If you use a traditional Windows machine, you may need to add a FireWire 400 port. Apple Macintosh computers have had [6-pin] FireWire 400 ports as standard for several years. You will also need to buy the FireWire cable - typically, the camcorder manufacturers do not include that cable. USB will be used only for transferring stills from the memory card.