Q. Why do some people stand all day on Yom Kippur, at least during Ne’ilah?
A. The practice of standing is recorded in the Shulchan Aruch (Orach Chayyim 619:5). An explanation in the Midrash says that destructive forces have no power over the people of Israel as long as they stand up against them, almost like Amalek being unable to conquer Israel while Moses was standing with his arms supported by Aaron and Hur.
Standing for lengthy periods is difficult but on Yom Kippur, when we are angel-like in that we need no physical or material comforts, we do not feel the hardship. Those who are unable to stand all day make an attempt to remain on their feet at least for the duration of Ne’ilah.
The Mishnah B’rurah (Orach Chayyim 623) points out that Ne’ilah requires an extra effort at concentration and holiness since this is when the Heavenly verdict is sealed and we dare not slacken in our penitence and prayer.