These caps are a great option for home brewers.
Rather than needing a double ball lock spear for each keg you can use these.
The pressure relief cap are great for those who want to naturally carbonate in the keg using dextrose etc without the need to worry about over priming. Should the pressure get too high in the keg (about 35 psi) the valve will release some to prevent any explosions or over carbonated beer (the kegs are rated to 60psi so it would take a LOT of over priming to make one blow up)
The carbonation cap is great for those who prefer force carbonating using a CO2 regulator. Simply screw it into the keg, attach your gas line or mini regulator and set to the desired pressure. Using a line splitter or a gas manifold you can carbonate multiple kegs simultaneously or have one with ball lock spear and be pouring from it while others with these caps carbonate.
For force carbonation you can use this helpful website to calculate the pressure required: www.brewersfriend.com/keg-carbonation-calculator/