Well, a compass always points to a world's magnetic pole. True North... which isn't quite the North Pole, but very close. But in the Grand Line, each island has its own magnetic field. Some very strong, some weak, some that appear and disappear - all mixing together - and the compass gets all confused. It doesn't know where to point anymore - True North is masked by all the interference from everything else, see?

So a Log pose cuts through all the interference to lock onto a single island, so you can safely sail to that island. And once there, it will reset to lock onto the magnetic field of the next island. It takes a little while to reset... sometimes hours, mostly days, sometimes years. But that's how people thread their way through the Line. The Log Pose will choose a path and they'll sail it.