Spreaders are flexible connectors that help maintain spacing on the legs of a tripod. With no spreader in place, the legs of the the tripod can move freely to allow leveling on extremely challenging terain.
The legs on a professional tripod are generally only attached at the top where they connect to the bowl that holds the head. The legs of the tripod typically have rather sharp spikes or points that hold the tripod secure when shooting outside. Press those spikes firmly into the ground and the camera isn't going anywhere. Tripods without spreaders are generally found only on film sets. Modern video cameras just don't have the mass needed to keep spikes anchored.
Spreaders come in two forms. One is a mid spreader and the other sits at the bottom of the legs and the function is the same. Spreaders allow you to use a professional tripod on surfaces where you can't use spikes to secure the legs. The spreader allows you to restrict the movement of the tripod legs relative to each other. Spreaders that attach at the bottom of the legs also protect floors from scratches.
Hope that helps.
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