Master Link Pliers are designed to help remove and replace master links in modern chain systems, although these are not needed when new, over time, dirt and wear make it increasingly difficult to open Master Links...
...Unless you have these.
If you value your time, these would pay for themselves in about two or three "THIS IS REALLY FRUSTRATING" chain link separation attempt sessions... :D
They also massively reduce the risk of bent/weakened links, by reducing the amount of time spent contorting and struggling with your chain :D
Put into link. Squeeze. Open chain. Once you're done, link the chain back together, and use these to pry the link back into it's fully locked position.
They're very high quality - Bike Hand are a Taiwanese bike tool manufacturer, and they produce superb quality kit. With a bit of care (don't store them in a swamp, maybe a lick of machine oil every year or so, etc), for the task they're intended for, they should last you an extremely long time!