A Promoted Block is something that may be offered with your Squarespace template. Some templates use them, others don't. If you see an image appearing above the rest of the content, then the template uses them. Otherwise, the image will appear in line with the rest of the post. Sometimes Promoted Blocks appear different from the other images in a post, for example, it may appear wider or even fullscreen. Or it may have some special aesthetic elements applied, such as a border.
Why use a Promoted Block?
If you have an image, video or gallery that you want to really stand out and draw attention, then you can put it as a Promoted Block, so it appears first and grabs the eye.
It can be larger than the rest of the images in the post, such as the photo of a flower that is shown here on the right side of this text block.