You can shoot a lot of photographs with your digital SLR camera, without seeing any signs of dust on your sensor. But when you shot with a small aperture like f/32 and have large unicolor areas like blue sky, you'll suddenly notice small dark spots in your images.
Look at the top center part of this image with a lot of blue sky (click to see final result on Flickr :)
I shoot a lot of landscapes at f/22 - f/32, and I have seen how my sensor have collected dust over the last months.
Today I finally pulled my self together and bought the Dust-Aid Wand. It was a scary experience to swap that cloth over my sensor this first time, but it helped :))
So I hope that I no more have to use the spot removal tool in Photoshop Lightroom (although it is a fantastic tool :)