This is a snap shackle (aka quick-release shackle)!
Its purpose is to be able to easily open the shackle even though it is currently under a load. Its designed to be tough, but It's still just plastic: so, it will only survive light to medium loads. Naturally, it’s better to have a metal one, but if you don't have one, then you don't have one :P
In my particular usage, I use a similarly (professionally) designed stainless steel shackle to stow our spinnaker halyard while during the upwind section of a race. However, the boat I'm on right now didn't have one... so I made one :)
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