doesnt the example of your granny show that things did go wrong in the good old days?
i went to parentcraft class which was a 4 hour class instead of multiple antenatal clases.
I found it interesting, you can ask questions, meet other parents to be, have a laugh, get your partners involved.
Of course it isnt going to give you the answer to everything but i found it really interesting to talk about what was available at my hospital, when to call hospital, how to cope at home in early stages and seeing a video did seem to help those there get an idea of what the stages are in birth.
your relative has obvously considered it well worth going, you consider it not and have coped without. If your baby wasnt presenting correctly then no antenatal class would help your labour but perhaps for some the advice given at classes can help get baby into correct position. Or maybe explain all the types of pain releif and which may make things harder for mother and baby during birth.
Nothing can prepare you for birth, youre right, but some like to have an inkling of what to expect.