The link posted by @ Rewap is well described about enterprise and implementation, There 1 - Many relation between enterprise layer and Implementation layer.
Means you may have more than 1 implementation layer in an Organisation.
And Pega wont suggest to instantiate Enterprise layer, but If you Instantiate Implementation layer you will achieve better reusability, and organised structure of class and Specific to the Unit.
**For Creating a class structure and Instatiation of Implementation layer "Inheritance" concept playes imporatant role.
For Better understanding I suggest you to go with SAE 1 and SAE 2 Pega coureses :)