Here is why that specific piece of paper was so significant:

: This paper leads them to discover that a room was built into the sanitarium that wasn't on the official public records—a room that "A" (Mona) was using to sneak in and out of the facility.

: Spencer realizes the blueprint is dated months before the construction of that wing actually began.

: While searching through a box of Toby’s things at the Apple Rose Grille, the girls find a blueprint for the "East Wing" of Radley.