While not a typical homebrew draft tubing size, there are some applications where 1/2" tubing comes in useful and you would want a direct connection without a series of adapters to your keg disconnect.
One application is filling your kegs via the diptube (liquid out) connection. This allows for a fast, splashless transfer with minimal oxygen exposure.
Another application is rigging a keg directly to a beer engine for English-style cask ales. The liquid-in port on many beer engines is 1/2".
Includes a 1/4 in. swivel nut, but if you're connecting to stainless steel or brass, you will also need a nylon flare washer to form a seal. Plastic quick disconnects (QDs) on the other hand, have a built-in washer so the additional nylon flare washer is not needed.