Black & White
While the other tools in the Effects category lean toward the fanciful, Black & White is downright useful. In days gone by, you'd have to load black and white film into your camera, so all of the photos you took on that roll were grayscale. (Kids, if the words "film" and "roll" are unfamiliar in this context, ask your parents.)
But now it makes little sense to shoot black and white in-camera, even though nearly all cameras provide the capability to do so. Why throw away all of that color image data? Especially since you can let Photoshop Express do the conversion for you.
The Black & White tool doesn't just convert the color image to grayscale. Each edit tile provides a different mix of values corresponding to the colors in the image. The results can be dramatic or subtle, depending on the look you're seeking.