First of all, DPI stand for “Dots per Inch.” The difference between 300 and 72dpi is the number of pixels in the image.
Retail sites require ebook and audiobook covers to be 72dpi or higher. The higher the dpi, though, the larger the file size. Most sites won’t accept a cover that’s larger than 2 megabytes, so it’s best to keep the dpi at 72 to avoid the retailer possibly rejecting your cover.
Print covers, however, must be 300dpi for it to print professionally. The images need more pixels for the color to transfer better to paper.
FYI – Whether an image is created at 300dpi or 72dpi, the Internet will always view the image at 72dpi. That’s standard for viewing any image online.