Artists need to learn to self evaluate. I can look at someone else’s painting and immediately tell what needs to be done to improve it, but it annoys me no end that I cannot do this for my own paintings. It helps to look at them different ways, which is the reason for the advice to evaluate with a mat. Other ways include looking at them upside down, looking at them in a mirror, and looking at them upside down in a mirror. These different perspectives of your own paintings can help you see more clearly what needs to be improved.
For me, taking a photo of my painting and looking at it on my computer makes faults glaringly obvious. I don’t know why, but problems just show up more clearly on the computer. In addition, you can change it to a black and white picture and check the values. By taking a photo, looking at it on the computer, fixing whatever bothers me, and doing this several times, I can end up with an interesting series of photos of the evolution of a painting.