The Ben Ish Chai (a”h), elaborates on the reason for Karpas, dipping celery in saltwater toward the beginning of the Seder. He cites Rashi’s (a”h) commentary to Masechet Pesahim where he explains that this is done in order to arouse the curiosity of the children. Although it is not uncommon to dip vegetables in dressing during the meal, it is unusual to do so at the beginning of the meal, even before we wash our hands and eat bread. This deviation from ordinary protocol will catch the children’s attention and lead them to begin asking questions.
The Ben Ish Chai explains that the mitzvah of telling the story of Yetziat Mitzrayim on Pesach should be done in the form of questions and answers, as indicated in the verse, “Ki Yishalecha Bincha” – “When your son shall ask you…” (Shemot 13:14). Therefore, one is required to evoke questions for the fulfillment of this mitzvah and this is the reason why we eat Karpas, to arouse curiosity so that the children will ask questions.
The Ben Ish Chai cites the comment of Rabbi Sheneur Zalman of Liadi (a”h), in his Halachic code, that since Karpas is eaten only to arouse the children’s curiosity, one does not have to eat a K’zayit of Karpas. Indeed, our custom is to eat only a small piece of celery, and not a full K’zayit.
The Ben Ish Chai adds that although the purpose of Karpas is to arouse the children’s curiosity, one must eat Karpas even if there are no children present or if a person conducts the Seder alone. Chazal did not distinguish between different situations and Karpas is thus required even if there are no children present at the Seder. By the same token, one must eat Karpas even if he knows that this will not inspire his children to ask questions.