In Book 17, many suitors are becoming aware of the vacuum of power that will exist with Telemachus gone. They plot to kill him. Penelope will have to choose at that point. For Telemachus, several advantages are gained by his assertion of himself. First, when the suitor who throws the stool at the old beggar (Odysseus) does so, Telemachus wants to get all fired up right then and there. But by hold himself together, he demonstrates poise, confidence, patience, and rage withheld. This is important because that rage will need to be released after Penelope's challenge.
At this point in the storyline, Telemachus needs to demonstrate that he is a man to be able to assist Odysseus in the upcoming battle against the suitors.