Who should definitly need orthodontist braces?

Braces. Difficult to answer. Let me better phrase it this way. Almost everyone can benefit in one way or another from orthodontics. However braces do remain to be an elective procedure because people can live with bad bites and crooked teeth.
Severe malocclusions. Some folks have very serious jaw and teeth issues that really require orthodontics-- it is no longer cosmetic but more medically necessary. Good examples and sever deep bites or overbites. The best person to determine is an orthodontist that can examine the patient. Go see one.
Braces. Difficult to answer. Let me better phrase it this way. Almost everyone can benefit in one way or another from orthodontics. However braces do remain to be an elective procedure because people can live with bad bites and crooked teeth. Nonetheless, the quality of lives are improved with braces whether it be for cosmetics, functional issues, periodontal issues, psychosocial issues, etc.
Malocclusion. Those that have severe malocclusoin and bone deformaties. When teeth are overlapped a lot and aren't able to function properly. Some people with these conditions with develope TMJ dissorders that are painful as well. Go see your dentist.
Many people. Orthodontic treatment (and a skilled orthodontist is best trained to provide this treatment) is advisable for anyone with a malocclusion (bad bite). If teeth don't look right, they don't fit right or work right, they are harder to care for, wear unevenly, and can be involved with jaw joint problems. Why don't you talk this over with your family dentist and ask for referral to an orthodontist.
Depends. There are a number of reasons to have or to not have braces, these can be evaluated by your doc. If asking whether you should have general dentist do them vs orthodontist, that depends on many factors such as issues to be correct, doc experience, removable aligner vs brackets. See your doc and discuss all issues, then make an informed decision.