They help understand all design compromises that were done to keep compatibility with C, which is was part of what brought C++ into the mainstream, but also the main cause of many of its warts.
[1] http://www.amazon.com/The-Annotated-C-Reference-Manual/dp/02...
[2] http://www.amazon.com/The-Design-Evolution-Bjarne-Stroustrup...
https://www.amazon.ca/Annotated-C-Reference-Manual/dp/020151...