Is there a functional reason, like helping with rainfall or snow, or is it more of a cost issue?
Are there decks that are solid pieces of bonded wood?
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To let rain water drain off the deck, and prevent pools accumulating in joints/cracks and causing mold and rot.
It also lets a deck dry faster to avoid slippery surfaces. Further, it spreads the dripping water over a broader area and allows it to soak into the ground underneath faster and more evenly, avoiding heavy drip lines at the edge of the deck.
While bonded wooden sheet decking could be theoretically possible, I have never seen it, and why would you want it?
The main reason for the gaps is to give wood room to expand and contract as moisture levels in the wood changes due to it being exposed to the weather. Without that spacing, the wood may expand and buckle, damaging the deck, and requiring repairs beyond the simple popped nail.
As answered above, usually the spacing between decking timbers is to allow for moisture to escape. Also some timber will expand when wet so the gaps do allow timber to expand/contract freely.