In the LS: UA, in his letter to Jerome Squalor, Jacques Snicket mentions that he asked Jerome to buy the penthouse apartment of 667 Dark Avenue, and to never sell it. What was Jacques' intention?
Signs seem to point out that the penthouse apartment used to be the location of a V.F.D. headquarter, which would explain why its elevator cage leads to the Baudelaire mansion (home to a V.F.D. family). Apparently it’s a leftover from the time volunteers kept getting outed by Geraldine Julienne and had to leave their headquarters in a hurry. Duncan was investigating this:
“This scrap says ‘apartment,’” Klaus said,
“and has what looks like half of a map. That might have to do with the apartment where we lived with Jerome and Esmé
Squalor.” [The Hostile Hospital, Chapter Five]
J : […] Since I have joined the organization, we have been forced to flee from a total of seven headquarters.
[The un-Authorized Autobiography, p.43]
Who know how much information and equipment we have lost, packing and unpacking so many times? [The un-Authorized Autobiography, p.51]
It turned out there was a floor above the penthouse. [The un-Authorized Autobiography, p.201]
So it’s entirely possible that a ton of old V.F.D. artefacts and lore got forgotten in the apartment once the volunteers left. When the Baudelaire orphans visit it, we see that the library is filled with Esme’s books on fashion, but Jerome used to live here as a bachelor and isn’t that interested in what’s “in” or “out”. Which means that Esme emptied the library of its original contents. Maybe she hoped to find important information related to V.F.D. in there. There’s an interesting parallel between Jacques investigating Monty’s library after his death and Esme’s interest in the penthouse apartment after the Baudelaire fire.
The other very good reason for Jacques to make Jerome buy the apartment is that it directly accesses the secret passageway in the elevator shaft. We don’t really know when Jerome acquired exactly, but it doesn’t really matter. Either Jacques was worried villains would use the passageway to burn down the Baudelaire mansion in a discrete manner, or he didn’t want the other side of the Schism to find out about it and realize a possible survivor had escaped the burning house. And it does appear that Jacques knew about this secret, since he was trying to get to the headquarters in the Mortmain Mountains (Link).
“But I’m innocent!” the man on the platform cried. “Please listen to me, I beg of you! I’m not Count Olaf! My name is Jacques!” He turned to the three siblings, who could see he had tears in his eyes. “Oh, Baudelaires,” he said, “I am so relieved to see that you are alive. Your parents /” “That’s enough out of you,” Officer Luciana said, clasping her white-gloved hand over Jacques’s mouth. [The Vile Village, Chapter Six]