Why you need to prepare a shop drawing for construction at site if the Construction drawing issued by the Engineer gives all details ?

No need to provide any plan / drawing for it is already provided by the engineer, all you have to do is to check the appropriate dimension / measurement and how do you implement the plan / project.

But if the tender drawings have all dimension and details as per site requirements , why we need to prepare a shop drawings??

bcoz construction dwg have no dimensions & details as per site requirements.