Access the many Java libraries available, it's the only Ruby interpretor with true threading (no GIL), integrates in the Java server ecosystem ( say with servlets via java-rack etc), upstream IT management approval (it's enterprise).
Interestingly, because it has true threading, it's actually harder to debug threading problems with JRuby than it is with MRI. MRI will tell you precisely where a deadlock happens, where JRuby just locks up. So it's good to have both.