Question

Why is it important to line up the digits in each place value position when subtracting?

Answer 1

It would not make sense if you were to do 
   34 
     28
The right way is 

34
28

Another example is that if you were to subtract by 1 digit by 2

78
  1

This is the wrong one

78
1

Answer 2

So that you don't subtract the wrong numbers and get the wrong answers. ,
123
-----
386
_____
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