Most of the time, the Generate mode will be good enough. This mode is much faster, which allows you to quickly experiment with different descriptions.
For backgrounds with tables, rocks, flowers, water, sky, and other objects that look realistic even if not accurate, the Generate mode can give you photorealistic images quickly.
If you use a reference image, the Generate mode will loosely follow the rough colors. Imagine a blurred version of the reference image. You then have the flexibility to describe the background you want, even if the background or objects are not in the reference image. The Generate mode will create images according to your description and match the colors in the reference image.
Sometimes we need a different tool for a different job. I personally use the Generate+ mode for three specific situations:
I want to generate objects that would look artificial if not accurate, such as animals, humans, and living rooms.
I have a reference image that I like and I want to generate images that are almost identical to the reference image but also include my product, such as necklace models.
If you use a reference image, the Generate+ mode will closely follow the reference image. Imagine a clear version of the reference image. You do still have to describe the background you want; otherwise, you will get weird results. The Generate+ mode will create images according to your description and the reference image.