No athens, you should only have to set it once.
For example, at the moment the pressure in Dublin is shown as 1038 mB. If you were setting it now you would turn the screw on the back until the needle pointed to 1038. After that the needle will move to indicate rises and falls in pressure.
There might also be a second needle that you can turn from the front. You can move this around as you please - e.g maybe just to mark a particular high or low point or if you want to watch trends.