A letter from a doctor that backs up your claim for disability most certainly can help your case. That being said, there are a couple of caveats that you need to keep in mind before you start counting too much on a doctor’s recommendations.
First, the doctor’s letter will have the greatest impact during the appeals process, specifically when you’re going before a judge. The initial disability examiners that look at your claim are bureaucrats. The system within which they work tends to make it much easier for them to deny a claim, so they will tend to go that way.
A judge takes a more objective look at evidence. Your doctor will have, obviously, more medical expertise than the judge and the judge knows that. They will tend to take doctor’s letter very seriously. Make sure you have the right kind of doctor’s letter, however. This means that the letter should be objective and not emotional and that it should give the judge detailed information that they can use to assess your eligibility.
The doctor should make an effort to give a useful assessment of how your disability will affect your ability to work and what kind of real limitations it means.