In the USA, a Captain's license is a license issued by the USCG (United States Coast Guard) for commercial operations. Meaning if you are going to receive remuneration for your services skippering or captaining a boat, you must hold the USCG Captain's license. There are two levels. 6 PAX (able to transport 6 passengers) and Masters. You are then also awarded a tonnage which is issued based on your previous experience in vessel operations. A typical lower-level license is a 6 pax 50 ton.
For Water-ways in the USA not controlled by the USCG you just need to adhere to the state laws. These are typically lakes and inland rivers.
Gaining the Captain's license is not an easy task because it involves a lot of Department of Homeland Security measures. For the operation of a sailing vessel, there is a lot of extra learning in the captain's license that relates to larger power vessels.
If you are just wanting to learn all you can learn and NOT do commercial operations then the USCG Captains license is not really the right thing to do. But the opposite applies - if you are going to do commercial operations then you absolutely must get the License (except some inland waterways).
If you are wanting to charter a yacht in the Mediterranean, the USCG License is NOT recognized as an acceptable sailing license. This is because there is no official assessment of your on-the-water skills to gain a USCG Captain license. Rather you should gain the NauticEd SLC.
Back to sailing vessels, there is absolutely no sailing training and competency training for sailing in the USCG Captain's license. There is a sailing endorsement to the license but about the hardest question on that test is "What is the big stick up the middle called?"
For all the reasons above, the NauticEd Captain Rank focuses on pure sailing knowledge and competence. And even if you plan on commercial operations you should still do NauticEd Captain Rank because of the knowledge and competence required to a responsible captain. The learning contained in the NauticEd Captain Rank is far far beyond the USCG training.
However, the NauticEd Captain Rank is NOT a USCG commercial license 6 Pax or Masters license nor is there any official credit gained towards the USCG captain license. The NauticEd Captain Rank means that you have completed the highest rank with NauticEd. Much of the learnings with NauticEd will help you pass the USCG Captain license exams.