1. why do people reserve a seat for a journey of this short length and then not sit in it?
2. why do people book in advance (and possibly pay booking/delivery fees) for a journey which costs the same on the day?
I find these reservations simply cause delays as people spend long periods of time looking for unreserved seats when boarding the train.
1. I think you've answered your own question, an advance ticket has to have a reservation if that train is reservable, but most people don't bother to track them down.
2. Remember that any advance ticket from Bath which involves a change at Bristol would have to have a reservation. So if you booked your Bath Spa to Birmingham Advance Single at ^13.50, you'd get reservations on the Bath to Bristol leg if the service was reservable as well as the
XC▸ bit from Bristol to Birmingham.