Why is it important to center a speciment on low power before attempting to focus on it at high power?

It isn't if you're good with the microscope.

But as far as beginners go, it makes things easier. You have a large "field of view" with the small objective (low power). This means you can get it in focus and center the specimen. When you switch to the high objective (high power) then your specimen will already be partly in focus and centered. If you didn't center it with the low objective you would have a difficult time trying to find it since it wouldn't be lined up and completely out of focus.

Binoculars work with the same principle. You don't look down at the ground with the binoculars then swing them up to the sky and try to find a bird. You look at the bird then you put the binoculars up to your eyes, already looking.

Good luck and have fun!