Question

why is it necessary to heat the oxalic acid solution before titrating it with the potassium permanganate solution

Answer 1

It isn't completely necessary, at the beginning anyway. The basic reason for heating is to increase the speed of the reaction between permanganate and oxalate since the reaction is so slow. One procedure that is recommended is to add about 90-95% of the permanganate required, with stirring (addition not too fast) at 25-30 degrees C, allow the pink color to disappear, THEN heat the solution to about 60 degrees or so (some references quote 60-90 degrees C) and finish the titration.