Rey Mysterio & Sin Cara have been teaming together on WWE TV as of late and that is for a few reasons.
First of all, WWE is still looking to build to a WrestleMania match between the two. WWE originally wanted the Mysterio vs. Sin Cara match at WrestleMania 28. The idea would have been that WWE would hand out Mysterio and Sin Cara masks to the audience and set the Guinness World Record for the most masks worn in a public gathering. Due to injuries, that did not happen.
WWE has decided to have the two team for now, partly to build a history with them and also to help both wrestlers in other ways. Rey Mysterio isn't 100% right now and the feeling s that he can be better protected in tag team matches while he gets back to 100%. Also, Rey teaming with Sin Cara is looked at as Rey endorsing Cara to the fans, which will help him get over and hopefully sell more merchandise.
There is also talk of the duo winning the tag team titles.