Spyvision allows you to use the remote like a torch, shining it over the screen looking for things to pull into the world with an upwards flick and then, typically, destroy for stuff. Usually it's a crate in among other crates that are actually visible. Sometimes it's a ticking time bomb crate – useful for killing enemies before you get close to them. And every now and then enemies themselves are cloaked and need to be pulled in before they can be attacked. At the end of the day, though, Spy Vision serves little purpose, and shouldn't really have made it into the final game.
Still, if you're a Wii owner you could do far worse. And for some crazies out there, the whole constant death replay entire stages thing will be like vanilla milkshake: aka as good as it gets.