A common question that gets asked is: What is After Effects used for?
The quick answer would have to be: A lot of different things!
The slightly more detailed answer would be that: After Effects is used for a variety of reasons by a host of different people and industries, it's a kind of jack-of-all-trades program that enables its users to create and experiment with moving imagery in some form or another.
Take this demo reel for example, found on an old blog post on ToolFarm. The video shows how the first version of After Effects was being used back in 1993.
It seems even in 1993 the question of what is After Effects used for was a difficult one to answer! As you can see, back in 1993 After Effects was being used for a variety of reasons even then: live action compositing, animation, and visual effects.
Over the years the feature list has expanded and the performance has of course increased, yet the program itself hasn't been cornered into one particular specialisation, and the reasons for using After Effects are as diverse as ever. It's a powerhouse of variety.
What other program, for example, could be used as the editing/vfx tool for a major Hollywood film like The Social Network AND at the same time be used to produce a fully animated cartoon like Archer? The sheer range of output from After Effects and what people create with it is a wonder in itself.
Beyond the moving image output, there's also countless programmers out there, including myself, who create After Effects Scripts and plugins that extend the capabilities of the software and help others create great things.
Perhaps then, the question: what is After Effects used for? Could be answered with a single word: Creativity.