I remember the first time I saw an image data struct that specified bits per pixel, image width and then also had bytes per row. I was a bit perplexed about why you'd store bytes per row if you could just compute it from bpp and width. Turns out you can fix/avoid a lot of performance issues if you can pad the data, so it's good practice to always save the number of bytes per row and use that in your mallocs/copies/iterators/etc.
For images, one of the main reasons to have separate bytes-per-row (usually called stride or pitch) is that it allows you to have sub-images that use the same data and stride as the parent image, but a different width and height.
