Question

Why we add a zero before a decimal point to represent it

Answer 1

Actually, you don't have to add zero before decimal point for it's representation. For any number like 1.27 or 3.33 , we have to specifically write the unit digit number. However, if we write a number as .7, without specifying the unit digit, it is understood that the number is 0.7. So we need not write zero before a decimal point.

If your question is why we add the unit digit as zero, it is to indicate that the given decimal number ( 0.7) is 7/10 more than zero