Cst purchase is mandatary or not?????

Respected Sir/ madam,

We know that input vat is eligible for get  input tax credit as per items listed in vat schedule  in west bengal. but cst purchases are not eligible for getting input tax credit. So my question is if we are not able to get any input tax credit against cst purchase then why should we make some purchases from outside of state. is there any restriction of local purchase in west Bengal?? or this can be controlled as per dealer wish to avail ITC??

kindly explain...........................

Regards

kamalesh Chatterjee