Blanking plugs are used in manifold valves when there's no manifold. When using loop bungee in sidemount setups a manifold valve with plug provides the most secure attachment point as compared to using some sidemount specific valves that have very little stem to secure your loop to.
Please ensure you've selected the correct side plug before ordering.
LHS plugs have reversed threading. When installing counter-clockwise will tighten the plug while clockwise will undo it.