Smoke and clouds!

Hah, but yeah, smoke, clouds, water, gases, etc... Ya know, like effects effects. Glows and blurs are more 2d comping software territory, like after effects.

Everything from the dust that kicks up when your cg character is running across a sandy beach to the swirling vortex that sucks the villian into oblivion, Houdini makes... good lord that sounds cheesy.

The final sequence of Spiderman 2, when Doc Oc and that burning orb are wreaking havoc in some warehouse was all done in Houdini. Collisions, fluids, soft bodies, etc. It might be easier to do a lot effects with Maya and it might be possible to get great looks (ESC's work in Matrix 2 & 3, Constantine), but Houdini is typically where most of the grad students with Physics degrees begin to drool. I don't do much plug-in writing, but I get to use some amazing plugins. Houdini always comes out being more robust and sometimes more confusing.

Bottom line: you don't need to know Houdini to get a job. Most companies use Maya, and understand that Houdini is better left for in-house training.