What does the back look like and why do I care?
The back generally looks like a thin, dark version of the front. All that bobbin thread, made visible against the plain white substrate.
I look at the back for the same reason an artist might squinch their eyes, or look at a picture upside down or in a mirror. It works to check on the balance of the piece. From the back, I can see if the picture has unintended empty spaces or overly busy places. I can make sure I am working to the edges of the piece. It helps me check my work without the detail on the front distracting me.
The only real problem with this check is when I am working from the back, using heavier decorative threads in the bobbin. Then I have to rely on mirrors to help me see compositional problems.