India is a sovereign country with many powers for states. Unlike many other countries, GST in India is implemented as duel tax system. Means, two types of GSTs are implemented in India. As you know, there are IGST, CGST and SGST. Each taxable transaction of goods and services in India attracts duel GST – CGST and IGST or SGST. So the main reason for duel GST is to account the share of state governments. Ie, SGST (State Goods and Service Tax) and the share of IGST (Integrated Goods and Service Tax).
So, an account of GST Tax payers is to be accounted separately. At present government insists separate GST registration in each state, if a person operates his business. For example, if XYZ company has business transaction of goods or services in Maharashtra, West Bengal and Tamil Nadu, he has to take registration of GST in all the three states. Means a separate GST registration number has to be obtained in each state, although the Permanent Account Number, PAN is same.
As per sub section 1 of section 19 of GST Law explains that every person who is liable to take a Registration will have to get registered separately for each of the States where he has a business operation and is liable to pay GST