To many people it simply looks cool and professional, but there are also practical choices that make this style appealing. One of the most compelling is that a white background adds a significant amount of light to the room. In a major conference where there are plenty of spotlights on the speaker and the room is professionally lit this does not make much impact, but in small and medium size events or in company meeting rooms you might notice that the brightness of modern projectors can be an issue.
One way to deal with this problem is to have a fairly constant intensity presentation where there are no slides that are significantly darker, but this can be monotonous. If you blank to black the screen when the images are not needed the impact is even worse. Back in the days of projection CRTs the screen image was so dim that this was never an issue. Now the kind of projectors you might find in any company meeting room can be bright enough to be a problem;
Apart from the distracting changes in light when slides change or the screen in blanked, a white background aggravates another issue. In many company and hotel meeting rooms it is hard for the speaker to stay entirely out of the projector beam all the time. When a slide is just a line of white text on a black background this problem is minimized. But when you have black writing on a white background you are much more likely to have some light from the beam in the speaker's face. This is annoying for the speaker and distracting for the audience.
You can reduce this effect another way by using a short throw or ultra short throw projector behind the speaker -- and possibly add two LED spotlights to brighten the speaker's face -- and you should definitely consider this for your own meeting rooms. But most speakers have more say in the making of their slides than in the equipment of meeting rooms so a white-on-black style is a simpler solution.
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If you would like personal one-to-one coaching to improve your speaking and presenting skills you can ask about buying blocks of hours either by email at firstname.lastname@example.org or by phone at 0046 73 089 44 75. I can also do group coaching, lectures and workshops for groups of people who need to become more effective presenters.