why should we put comma in indian system first time after three digits and then after two digits?

In Indian number system, commas are used to mark thousands, lakh and crore. The first comma comes after hundreds place that is, three digits from the right. After this, we apply commas after two digits. The second comma comes two digits later that is, five digits from the right. Similarly, the third and other commas. 

It is done so because in any number system, we add commas according to the periods. In Indian number system, we have periods as Ones (having three sub columns ones, tens and hundreds), Thousands (having two sub columns), Lakhs (having two sub columns), Crores (having two sub columns) and so on. So, we apply comma to separate these periods in the manner 3, 2, 2, 2 and so on.