PHP is an interpreted language. So, it suffers from performance disadvantages as compared to other compiled languages such as Java or C/C++. So, Facebook created HHVM, an alternative PHP interpreter (alternative to popular Zend) that can optimise PHP code while still acting as an interpreter. So, obviously the main benefit of using HHVM is that you gain a lot of performance benefits. Read how HHVM made editing in WikiPedia faster.
The other benefits of HHVM are :