Dated 20th Jan 2006
After so many replies telling you how and when they use it, still no one has given you the answer to your question.
I don't own a FZ5 an d can only guess that it's probably the "focus lock" button.
If my guess is correct that it’s a focus lock button, the main difference between locking focus by pressing the shutter release button half way and using the "focus lock" button is that the former locks both focus and exposure while the latter locks only the focus and you can adjust the exposure either manually or through auto exposure. They are similar in the sense that they both reduce focus lag by pre-focusing first and this is useful when timing is important. (such as taking pictures of fireworks and subjects in fast action).
(In some cameras, through camera setting in the menu, you can set whether the button is locking focus or locking exposure.)
In some situations, focus lock is used instead of the shutter release button because the light condition of the scene will change later. If you have pre-empted by pre-focusing through pressing the shutter release button half way and the light condition changes after that, the image captured will be under-exposed or over-exposed. A good example is taking pictures of fireworks : If you’ve pre-focused by pressing the shutter release button half way, both the focus and exposure in locked before the scene get brightened up significantly by the fireworks and therefore you will get grossly over-exposed fireworks captured. If the focus lock is used for pre-focusing instead, assuming the camera is set to some kind of auto exposure, the camera will only adjust the exposure by taking into account the fireworks lights which have appeared when the shutter release button is pressed.