Additional ticket costs for what?

When booking a flight on mango, there is an option to pay R150 for the Lowest plus thats supposed to give you a free standard seat reservation. There is really no need to pay this extra R150 as you do not got a free seat. When i went into my booking now for my flight it showed no reserved seat. I then went to the Add extra and it showed the R55 which one would pay for a seat however the all the seats showed blocked out. Over half an hour on the phone with Mango and 3 consultants later, I still am not sure why I had to pay the additional R150 with no seat. Whats the point if I have to wait for check in anyway and then end up in a middle seat in row 29??