The chief function of the ECP is to place constraints on the movement of categories by the rule of move α; it effectively allows a tree structure to "remember" what has happened at earlier stages of a derivation, and it can be seen as GB's version of the older derivational constraints.
The core purpose of ECP involves setting boundaries for category changes using the move α rule; it helps maintain a tree's memory of earlier derivation steps, akin to older derivational restrictions in GB.