Reflections on leather. Leather reflects light. A shoe photographed on a coloured surface picks up that colour in reflections. The cutout preserves the reflections, which can look wrong on a white background. Diffuse lighting minimizes this.
White soles on white background. If you shoot a shoe with a white sole against a white surface, the model has low contrast at the sole. Shoot against a light grey or blue surface instead.
Laces merging with background. If the shoe has long laces that extend beyond the sole, and the background is similar to the lace colour, the laces can be lost. Use a contrasting background.
Shoes photographed together. If the photo has two shoes (left and right), the model will treat both as subjects. The result is a cutout with both shoes. If you want only one, crop the image first.