When a deck board is cut down the middle to a specific width, and installed adjacent to the house, it's sometimes called a house rip board. When the deck boards are laid out perpendicular to the house, starting at the front of the deck and working towards the house, the last board is often not going to fit adjacent to the house and must be cut down.
Having this ripped board next to the house allows for the rest of the deck boards to be laid down with consistent spacing. If the front of the deck is not parallel to the house, the house rip also allows for the different in angle to be accounted for by having the rip cut narrow on one end and wider at the other end.
Ultimately, this rip is what covers and hides the inconsistencies in the deck framing and layout, and having the cut edge hidden under the home siding makes a clean transition and keeps compromises on deck board spacing and layout to a minimum
