Percentages is the best example of why it's so useful. For value A at 41.5% and value B at 58.5% their sum is equal to 100%. Rounding up would make them 42% and 59% respectively, thus 101% in total. Round half to even ensures the final values of 42% and 58% sum to 100% as intended.
I presume you mean the "drawsAsynchronously" property. I'm extremely curious, does it really push the rasterization on the GPU? I mean, do you have shaders written, that do all the stuff that CPU normally does? Bezier paths, clipping, stroking, filling?