The content sits at the edge of the slide. A background-color on this slide would sit behind the text.
The content sits in a card that floats above the background. A background-color on this slide sits behind the card.
The yellow background fills the full viewport, while the content stays cleanly inside the white card.
The background can be any color or image without affecting the readability of your content.
A photo fills the full viewport behind the card, keeping the content easy to read.