What are the advantages and disadvantages of buying from an authorized dealer?
The clearest benefit that a brick and mortar authorized dealer can offer you is the complete manufacturer’s warranty. Unfortunately, this is often the only benefit.
For many brands, these dealers will have access to the entire line of that brand, however, due to limited display case space, you will usually just see a small portion of the different styles. If they do not have the watch you wanted, they would have to order it for you, often with a non-refundable deposit... This puts you in the position of ordering a watch, sight unseen, and without the protection of the return policy offered by online dealers.
If the watch is at the main US distributor, they should be able to get it in for you within a couple of weeks, but if the watch needs to be ordered from Switzerland, you may end up waiting a while. Contrast this with the overnight shipping offered by online dealers.
You could also spend your time searching local dealers for the exact watch you are looking for, but unless you are in a large market, there may only be one or two dealers in your area that carry the brand.
Authorized dealers should have more information at their disposal from the manufacturers, but that does not mean they absorb it. Nothing can beat having a great in-store experience where you ask a dozen questions, and the salesperson honestly and knowledgeably answers all of them. That is what you should expect when you go into these stores and pay full retail. However, salespeople are often on commission, so honesty can get left to the side, or they'll just make something up if they don't know the answer.