Normalize does the following for each channel:
image = (image - mean) / std
The parameters mean, std are passed as 0.5, 0.5 in your case. This will normalize the image in the range [-1,1]. For example, the minimum value 0 will be converted to (0-0.5)/0.5=-1, the maximum value of 1 will be converted to (1-0.5)/0.5=1.
if you would like to get your image back in [0,1] range, you could use,
image = ((image * std) + mean)
About whether it helps CNN to learn better, I’m not sure. But majority of the papers I read employ some normalization schema. What you are following is one of them.
Hope it helps.