You have your grid in place and the drawing is done, maybe you have already started painting. At some point you want to check how close you are to the reference. Are the proportions right? Did something shift slightly? Does something feel off but you cannot quite tell what it is?
Uploading a progress image and comparing it to the reference makes these problems much easier to spot.
In the Progress images section, click the + button.
You can:
The image will appear on top of your reference.

Make sure your progress image matches the canvas frame and the reference image.
Only the part inside the frame is visible, so anything outside will be cropped. Adjust your image until it lines up as closely as possible.
If needed, adjust your frame first so it matches your painting proportions.
Learn how to adjust canvas frame / crop image
Use the transparency slider to make the progress image partially transparent.
This lets you see both:
You can quickly spot:

Turn the progress image on and off.
This makes it easier to compare:
Switching between the two often reveals mistakes immediately.
You can upload several progress images.
For example:
Turn them on and off as needed and adjust transparency to compare different stages of your work.
A good photo makes comparison much easier.
Small perspective errors can make alignment harder than it should be.
Try Paint From Photos and simplify your painting process right now!