Builders need to know the slope (aka. pitch) of roofs for architectural framing loads & snow load purposes; the slopes of decks & patios for good water drainage, so foundations & basements are not flooded or eroded. You need to be aware of the your property slopes for the same reason, good drainage. The same re ease of mowing & yard maintenance. Driveways too, esp in winter when it's icy. You don't want you or your car to slide uncontrollably into the street. Re plumbing, bathtubs & shower floors must be properly sloped, as well as septic drainage lines.