I only needed one for eval. I was told its to discover any hidden issues (eating disorders or mental blocks) that might impede your success. Any disorder needs to be addressed before surgery.
Psychological issues with food don't go away because you had surgery. Most of us have had to change our relationship with food post op. But some have more serious issues, and that's what they're checking for. To make sure you have the support you need to succeed in this.
I have often wished that they had post op bariatric counseling. Instead, I found you guys!! lol