If you have compiled wxWidgets in Unicode mode, the wxChar type will become identical to wchar_t rather than char, and a wxString stores wxChars. Hence, all wxString manipulation in your application will then operate on Unicode strings, and almost as easily as working with ordinary char strings (you just need to remember to use the wxT() macro to encapsulate any string literals).
But often, your environment doesn't want Unicode strings. You could be sending data over a network, or processing a text file for some other application. You need a way to quickly convert your easily-handled Unicode data to and from a traditional 8-bit encoding. And this is what the wxMBConv classes do.