1. Question: Why does one have to stand with the baal korei when he is saying the blessings on the megilla for you but he does not have to stand with the baal tefilla when he is saying the blessing for the omer for you?
A. Rav Dovid Cohen based on the teachings of the Chazon Ish said that the concept of “listening is like saying” is not just that he is saying it for you but by listening it’s as if you are saying it yourself.
B. Part of the mitzvah of saying the omer with the blessings is standing, just as hearing what’s being said is part of the mitzvah.
C. One can technically sit when reciting and definitely listening to the megilla but should stand when saying the blessings.
D. The concept of “listening is like saying” (שומע כעונה) applies to the mitzvah that the other is helping you fulfill but, for example people should stand during chazaras hashatz according to the Rema 124:4 out of respect for the blessings the chazzan is reciting when repeating shemone essay out loud and by doing so it’s as if the congregation says it based on “listening is like saying”.
Answer: By the omer since standing is part of the mitzvah so if the chazzan stands for you then you fulfill the mitzvah but by megilla the standing isn’t part of the mitzvah, you can really sit but out of respect for the blessing everyone should stand so it does not help if the one saying the megilla stands for you (See Dirshu Mishna Berura 690 footnote 2).