The alpha channel is not only on or off. Partly transparent pixels carry the soft edges of a cut-out: hair, fur, sheer fabric, glass and a soft shadow. Formats and settings that know only fully visible and fully empty pixels, such as GIF or a palette PNG, turn those edges into a jagged fringe that looks pasted on when the product is placed on a new background.
Edge pixels also keep some colour of the background they were cut from. A product cut from a dark or coloured backdrop and placed on white often shows a thin dark or tinted rim. Viewing the cut-out on black, on white and on a strong colour shows the rim at once, and the colour cleanup or defringe option of an image editor removes it before export.
When a channel does not support transparency, or converts uploads to JPEG, the alpha channel is dropped and the empty area becomes a solid colour, black on some systems and white on others. A soft shadow built from partly transparent pixels can turn into a grey patch. Flattening the cut-out onto the background the channel asks for, at export, keeps the result predictable, while the transparent master stays for design work.