It's not just protecting the few people who misguidedly believe they are trans (there's a lot of doubt within trans individuals about themselves and what they are), but also protecting the doctors from assisting those few people who fall into the previously mentioned category. The doctors have pretty much nothing to go on except the word of their patient.

Gender issues are not the only area of medical treatments where doctors employ assessment of clients undergoing emotionally difficult and voluntary medical treatments, it's for example standard practice for treatment for infertile people to require a psychosocial assessment before beginning treatment (which requires hormone therapies and potential surgery procedures).

As the requirements for gender reassignment surgeries have been lowered, more people express regrets about having the surgery. Some have even filed a malpractice suit, stating they didn't get adequate assessment before their referral for the treatment.

In a consumer-driven market you could say transsexual treatment is a commodity and gender dysphoric clients desiring treatment are vulnerable to physicians without adequate training who might bypass any assessment process.

Many trans people have a mental condition such as anxiety and depression. Some are even chronic and persistent. They may be because they're in the wrong fucking body, but there's also a chance they're there for other reasons. They can manifest in limitations that may interfere with their ability to make informed decisions about their medical treatment. Some are indeed suffering from severe gender dysphoria and are emotionally distraught and in tremendous psychological pain. In this case there's a chance they are not educated and informed consumers. They need more than referrals to the hormone doctor or even good psychotherapy, they need to be protected from making decisions that they may not be able to handle and will negatively impact their quality of life.

A year of being in the role can help bypass any such doubts, while also possibly helping the person feel better.