Long ago, I would say that the first reason for Groovy was its learning curve. That is still important (see below), but my new first reason is: Grails.
I imagine most readers are familiar with Grails: a Rails-influenced web framework that stands on the shoulders of giants -- namely, Hibernate and Spring. The reason I put this item up front and center is that you can make money with it. There are real jobs to be had here, folks. This is a powerful framework with plenty of momentum.