Everything in Houdini is really procedural (like max taken to an extreme) which is conducive to procedurally driven problems like creating a city, particle systems, physics etc. Everything play really nicely with one another. A physics sim that affects a cloth sim which affects a fluid sim which affects the animation of a blade of grass. That kind of thing.
Think particle flow wrapped inside of cloth wrapped inside of reactor wrapped inside of reaction manager.
It's not very approachable--although I hear from my animator friends that its tools are excellent for character animation as well.