In various levels, you can find a viewfinder/binoculars that look like this: 
What are they used for? You can stand behind them and look around the level, but is that all that they are for? That strikes me as a pretty strange thing for Nintendo to have added.
Best Answer
When you look through them you are generally supposed to be looking for a toad, the toad will then throw a 1up or a star coin down for you. I think they were just added to take advantage of the gyroscope controls the 3DS has built in.