Joab had not helped Absalom try to become king, but he had helped Adonijah. So when Joab learnt that Adonijah had been killed, he ran to the sacred tent and grabbed hold of the corners of the altar for protection.
Once a year Aaron must purify the altar by smearing on its four corners the blood of an animal sacrificed for sin, and this practice must always be followed. The altar is sacred because it is dedicated to me.