Bcoz, The Taxpayer can make Sales from the Other State and can be charge "CGST & SGST". SGST for the State tax and it's to the state where you registered.
EX. Mr. A registerd in U.P. and he made sales in Gujrat. Why he register himself as Casual taxable person in Gujrat. Whereas he has option to issue Bill of IGST
@ Sachin You are correct Mr A can supply goods to customer in Gujarat by charging IGST, but cannot Install or Open a counter or exibit & Sell his goods in Gujarat , he has to take CTP registration for the same .