Later, by the Mitzvah of Kidush Levanah and lighting the Menorah (page 59) it says that our custom is to wear agartel. Why only these two?
[Side note: The Sefer Haminhagim was written by Rabbi Groner. It's known that almost everything was told to him (or shown to him) by the Rebbe.]
One way to understand this can to find a connection between the above mentioned two Mitzvos. Both are done during (or near) nighttime.
The reason for wearing a gartel is to separate the head - the upper half of the body, from the lower parts. At night we need extra protection.
True, we need to go out there, but safeguards must be set!