Why pre book a seat when they put you where they want to.

My return flight from Cape Town to Lanseria (MN452) was delayed by over 2 hours. Whe I arrived at Cape Town airport I was informed that my seat number had changed to 16c, after having reserved and paid extra for seat 4d. I always sit in this seat as I suffer from claustrophobia and find this the best place to sit. I specifically booked my flight 3 months ago for the very reason that I could get in early and pre book my seat at a fee of R70 each way. Well, why pay extra if Kulula just ignores this request and seats you anywhere on the aircraft. The agent at the airport could not give me a reason for the change. I had to sit in the middle of the plane that was packed to capacity and therefore had an extremely uncomfortable flight, as well as having to sit on an extremely hot plane whilst a dispute about fuel quantities, that took over half an hour, was sorted out. Never again!!!!