1)They are his advisors so they basically give suggestion, comments, ideas (and ADVICE) to the president, influencing his decision.
2) This also shows that our government also follows the "unwritten constitution" Our founding forefathers, did not include in the constitution, for the presidents to have a cabinet. Having a presidential cabinet was not something that was LITERALLY written in the constitution. This was more of a precedent, that was followed ever since President George Washington decided to set up his own presidential cabinet. Based off his experiences, it was helpful. and it worked. Therefore, other presidents played follow the leader, and had also used the presidential cabinet, as well. This shows, that our government does not ONLY follow the written constitution, but also the UNWRITTEN CONSTITUTION which is adopted through precedents.