The main reason you'd quilt from the back is so you can concentrate on the free motion design and not get distracted by the pattern of the top. Often if freemotion quilting a top that has a specific pattern or a regular design in the piecing, you inevitably start following the pattern in the fabric and your FMQ suffers because of it.
If your tension is correct, you shouldn't be able to notice if it was quilted from the back or the front.
It also allows you to use thicker threads in your bobbin - the ones that break all the time when they go through the tension discs and the needle. Sometimes they work better when they're in the bobbin, so Upside-down quilting is the way to get the nice thread on the top!
There is no wrong way to quilt - just design opportunities!