If nothing is selected on your slide, the Grayscale View toolbar settings apply to the slide background. But if you select an object such as a shape, the grayscale settings apply to that object alone.
So if you have an object on the slide that has no purpose in the printout, such as a video file or navigation arrow, you can select that object and choose the grayscale setting Don't Show to hide it from printing. The animation illustrates this.
If you apply the setting to a single slide object, that object only will print accordingly. Suppose you've created a chart on one slide and selected the colors for it. You can use Grayscale View to make sure that those colors are clearly distinct in a grayscale printout. This also assures that the chart is accessible for people who are colorblind.
Or suppose you're designing a logo for a client, and you want to make sure it looks good on any printer — color or black and white. So, you check with Grayscale View as you're designing, and if you need to, choose an appropriate grayscale setting just for the logo. If the logo is on the master slides, apply the grayscale setting to the logo on the master slides, and it will apply to all slides in the show when you print in grayscale or in black and white.
Note If you've hidden an object and want to display it again for printing, select it, and on the Grayscale View menu click Automatic.