Alex, this is a very general question, so it's very hard to give you a specific answer. What exactly is it that you want to do with your career?
After Effects is software for motion graphics, compositing, and visual effects. It is used by individuals and studios all over the world for these tasks.
There are other compositing and effects systems, but I don't think there are any other serious contenders for advanced motion graphics work.
AE touches every project that I do in my studio now at some point -- I certainly couldn't do the work I do without After Effects.
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I've gotten some new opportunities as a graphic designer that I would not have otherwise. Instead of just working on basic page layout, my boss now looks to me as the "go to" guy for general video work.
Being available to help and having a wide variety of competencies has made me a more valuable resource. You just never know where you'll find your niche and have a chance to be a stand-out member of your creative team.