Strip the icons and the URLs out of a sitemap and you're left with boxes connected by lines, arranged in a hierarchy that thins out as it moves from a single entry point toward many specific destinations. Strip the furniture out of a floor plan and you're left with roughly the same drawing. This isn't a coincidence dressed up as an analogy — both diagrams solve the identical design problem: how does someone unfamiliar with the structure find their way from the entrance to a specific room without a guide.
The lobby problem
Architects spend disproportionate care on lobbies because a lobby is a decision point disguised as a room: from here, where does everyone go next, and how do we make that obvious without a person standing there directing traffic. A homepage is a lobby. The same design questions apply — what's visible immediately, what requires a choice, what's deliberately hidden until it's needed — and the same failure mode applies too: a lobby, or a homepage, that tries to make every destination equally visible succeeds at making none of them findable.
Depth as a design decision, not an accident
Architects measure a plan's efficiency partly by how many decision points separate the entrance from any given room. Information architects measure a sitemap by click depth for the identical reason. Both disciplines know that depth itself isn't the enemy — three well-signed decisions beat one confusing one — but undisciplined depth, added a page at a time with no one looking at the whole plan, is how both buildings and websites quietly become impossible to navigate.
A sitemap you'd be embarrassed to build as a physical floor plan is a sitemap worth redrawing.
It's a useful test, and an uncomfortable one, for any site that's grown organically for a few years without anyone stepping back to look at the whole plan.