You will probably always want to display colours on the screen but it is very common to print in black and white.
To display a presentation in black and white select (View > Color / Grayscale)
The Pure black and white option was originally for people to check the presentation looked alright before faxing it.
This is less important now.
When you choose Grayscale the Grayscale toolbar will be displayed.
This lists a variety of different grayscale and black and white options.
Select the object and choose the most suitable setting.
This setting will be remembered when the presentation is printed to a black and white printer.
Changing the greyscale setting will not affect the colour displayed on the slides (or during a slide how).
It will only affect the slides when they are printed in black and white.