Why C++? There will be as many reasons to use C++ as there will be readers of this book.
You may have chosen C++ because you have to support a C++ project. Over the 30 years of its lifetime there have been millions of lines of C++ written, and most popular applications and operating systems will be mostly written in C++, or will use components and libraries that are. It is nearly impossible to find a computer that does not contain some code that has been written in C++.
Or, you may have chosen C++ to write new code. This may be because your code will use a library written in C++, and there are thousands of libraries available: open source, shareware, and commercial.
Or it may be because you are attracted to the power and flexibility that C++ offers. Modern high-level languages have been designed to make it easy for programmers to perform actions; while C++ has such facilities, it also allows you to get as close to the machine as possible, gives you the (sometimes dangerous) power of direct memory access. Through language features such as classes and overloading, C++ is a flexible language that allows you to extend how the language works and write reusable code.
Whatever your reason for deciding on C++, you have made the right choice, and this book is the right place to start.