With sublimation, it is absolutely necessary for the substrate you are printing on to be white or light colored in order to image the substrate. Sometimes substrates come with just a white patch, which in many cases helps to mask that it is a sublimated item once sublimated. In some cases, it may be like an iPhone case where the sublimatable piece is an insert and imaging right to the edge of the insert is better.
Most sublimation users will print an image onto the entire white patch to mask the patch after it has been imaged. There are two reasons why lining up your image with the white patch would be challenging
1.) It is difficult to measure the exact area of the white patch. If your measurements are short a millimeter, it will show a thin white line after sublimating.
2.) Even if you get the exact measurements, it is difficult to align your image exactly to the white patch
The solution is to bleed your image!
Bleeding also allows you to print all the way to the edge of a sublimatable item!