Tiresias warns Creon to heed the seer because Tiresias knows the consequences of defying the gods and ignoring prophecy. By warning Creon, Tiresias is trying to save him from the calamity that awaits him if he continues on his current path of defiance.
Tiresias warns Creon to heed the seer because Tiresias knows the consequences of defying the gods and ignoring prophecy. Creon's refusal to listen to Tiresias and his arrogance in thinking he knows better leads to dire consequences for himself and his city.
In the text, Tiresias tells Creon that he has angered the avenging spirits of Heaven and Hell by leaving a corpse unburied, which is a sin against the gods. Tiresias predicts that Creon will suffer the same fate as punishment for his actions.
By warning Creon to heed the seer, Tiresias is trying to save him from the calamity that awaits him if he continues on his current path of defiance. Tiresias knows that the gods' wrath cannot be escaped, and it is in Creon's best interest to listen to his advice.