Transparency is stored in an extra channel of the file, the alpha channel, which records for every pixel how much of it is visible. PNG and WebP can carry that channel; JPEG cannot, and saving a transparent image as a JPEG fills the empty area with a solid colour, usually white or black.
A transparent product image can be placed on any colour, in a banner, a collage or a page with a tinted background, without a white box around it. For a marketplace main image the requirement is normally a white background and not a transparent one, so the same cut-out is exported twice: once transparent for design work and once on white for the listing.