DDoS attacks come in all shapes and sizes - not every provider is equipped to handle every possible attack! Some attacks are very complex and require managed mitigation and pro-active approach to counter effectively. No provider can promise you "unlimited" ddos attack protection because of obvious bandwidth limitations.
There are overrage charges when you exceed the allocated quota - bigger the attack, bigger the bill. When you hit the bandwidth cap, they take your website offline which effectively means mission is successful for attacker.
In this case, DDoS protection can help you save your precious data, loss of revenue, and lost work hours and productivity. We can do this by reducing attack surface area and allowing users to use the site normally as much as possible, even during the attack. As the DDoS protection is in place, we can also know about the enemy and prevent future attacks or prepare for them in advance.