The timezone offset feature in Lightroom is meant to compensate for the time difference between the camera (set to local time) and the GPS device (set to UTC).
One could set the camera time to UTC and never think twice about the timezone offset but this is kind of weird for non geeks. In practice, this wouldn't work well since camera clocks drift quickly and need to be adjusted frequently. If you need finer adjustment of the offset between camera time and GPS, e.g. if camera time was set incorrectly, I recommend using Jeffrey Friedl's plugin Geoencoding Support.
Cheers,
Fabien