Simple things... like changing a tire, jumpstart, or some gas... etc... are free Complex things such as changing a part will cost you money... if they can fix it on spot and get you going... like belt broken... you pay for belt and labor at 1.5 times normal rate. Sounds pricy but how much is your time worth?
If they can't fix it, then they will tow it to their shop... that will cost you too. But standard labor rate rather than 1.5 times.
AAA in USA is good for basic things... overlaps what MB offers in Roadside assistance. But they don't fix your car on spot...towing with basic membership is 5 miles max and extra will cost you. Deluxe membership will tow you up to 100 miles. However, they are not totally reliable... like recently, my friend got rear-ended badly... called AAA... and the towing company refuse to tow his car.
MB Roadside Assistance will not strand you on side of the road regardless how old your MB is or which owner you are. On newer cars that is still under their warranty, they will even cover your towing expense, put you up in hotel, give you a loaner car, and reimburse you some expenses while your car is being serviced... it is pretty awesome yet they are betting that majority of their car will not break down on major road trips so this service is rarely used.