Question

How does understanding place value help you to subtract across zeroes?

Answer 1

When you are subtracting over zeroes, you may need to carry over.  This being said, you may need to take some from a previous number and put it into a zero, and continue carrying over until you can do what you needed to do.  If you don't know the place value of the zero in mind, you will not end up completing this correctly.

Answer 2

When you subtract with bigger numbers, then you sometimes you have to carry over. But when you carry over, your work gets messy. It is also helpful when you know place value because it you do this:
   25
-    10
You would get it wrong. This is the correct way:
   25
-  10