This is not neccessarily true! You can make decent text based RPGS (many games for that matter) in a single file using structures only, not classes! PM me for a source code of a pretty cool basic game if you want creator of thread.
later
I did not say anything about games, but in C++ it's best if you know classes along with file i/o because it simpilfies a lot of things.And they are the basics of C++.
Now structures and classes are complimentary it's really your choice most of the time.But usally if you have experiance using classes, for games many classes will be used.