Why do you line up the decimal when adding or subtracting decimal numbers?

Yes, When Adding and Subtracting decimals you should line up the decimals. You need to keep each place value in the same column, the same as when you add whole numbers you keep the ones column lined up, same for the tens, etc. Lining up the decimal points makes sure that the tenths column is kept together, same for the hundredths, and so on.