Because it helps to see which one is greater, For example: if you line up 0.079 and 0.127 you can easily tell that 0.0127 is greater because it is closer to a hole number. Another example: 0.1 and 0.3, obviously 0.3 is greater. in conclusion, the reason you line up the decimal points is to make it easier to see which numbers are greater, which helps you compare and order easily!