Consider if a rocket ship was just 1 degree off right after take-off, by the time it entered orbit, it would miss its target by miles.
Course deviations are usually not very blatant; they tend to be subtle; we're just a hair off our correct course. Yet if a considerable amount of time transpires without correction, we find ourselves no where near our intended destination.
What do our sea, air, and space-faring vessels all have in common with sheep? Without guidance, they tend to wander. We have computer programs to monitor our flight paths and provide information that will correct our course.
Sheep have the same thing: but in their case, they're called shepherds. And we are typified in Scripture as sheep; we likewise need a shepherd, only a very GOOD Shepherd, because as much as sheep can wander, we wander further still!