To my understanding, a porch is pitched to help prevent standing water or puddling on the porch, and make sure water runs away from the house. Kind of same reasons to pitch roofs, gutters, and shower floors.

how much pitch there should be might depend on the materials and size of the porch, someone better educated than I about the specifics is likely to pop up and tell you. But I’ve seen porches where the slope isn’t noticeable, and others where it was very noticeable, like some of the old farmhouse porches.

Yes, porches and decks should have a slight slope to them to allow water to drain off. This is especially important for porches, which typically don't have any gaps between the boards for water to drain through.

We recommend a slope of at least 1/4" per 10 feet, but a tongue and groove porch may benefit from an even steeper incline.

Whether the substrate is concrete, rock, or soil, angle the boards so that they slope away from the house for the exact reason of helping shed the water away from your home and your foundation. We recommend a pitch of roughly ¼” for every ten feet. If using standard decking the space between boards should range from 3/32” to ¼” depending on whether you are face screwing or using a stainless steel hidden clip fastener system like the Eb-Ty Precision Clip for Ipe. If going with tongue and grooved porch flooring you can keep roughly the same angle of pitch, but make sure there is a roof over it.

Just received these photos from a project completed last week. Owner is planning on going back and covering the wood seawall with more cumaru…. Relevance to this thread is if you look below the stairs you'll see a slight arch/slope to the deck for shedding water. For this application it works great and is actually how shipwrights would've historically built the deck of a ship... down the centerline so water sheds left and right (or port and starboard as the case may be). Our owner's family's house built in the 1800's has a front porch built exactly this way (house was built by ship carpenters) and after 130+ years has never had water nor foundational problems. Walking on it you don't notice the slope unless you look at the shingles along the edge that abuts the house and see that on the left and right side ends of the porch it's about a full shingle line lower than at the center. That said, we still encourage sloping the deck or porch AWAY from the house/foundation.

Cumaru Deck, Dock Landing and Steps Milton, FL · More Info

There's no such thing as 1/4" in 10 ft. That is basically level. Maybe 1/4" per foot, so an 8 ft. wide porch would slope 2".