Federal prosecutors may have executed this search warrant for the premises of an attorney because Cohen may be a subject of an investigation. But, as an attorney, he is also engaged in the practice of law on behalf of clients — clients whose privileged materials are now in the possession of federal agents.
Cohen's office potentially contains documents and communications to all his clients — not just Trump — that are privileged and confidential. The documents must be reviewed for privilege claims, and privileged documents are supposed to be returned to the attorney from whom they were seized.