Chains can take a real hammering while you ride and sometimes they break. It's worth carrying a spare quick link so you can sort yourself, or a friend, out when this happens. KMC Missing Links compatible with the equivalent SRAM and Shimano chains. 

Here's  KMC Missing Link for 12 Speed chains. KMC says that this link is not re-usable - sadly they do not offer a re-usable version for 12 speeds yet. 

Some riders even swear by carrying two of them :)