Is it important to summarize and take agreement from customer over what you have discussed with him/her at the end of the sales call.If so why?

Yes it is important, as it is necessary for further discussion, negotiation and agreement, and also to avoid any conflict.

Yes as a buyer I do it regularly on e mail and hard copy with signature of sales manager - copied to assistants for filing and concerned sales manager. 

Very useful tool to record discussion / negotiation / agreements / issues and for follow up