imread() reads in the image. It seems to be a uint8 image. They didn't have to do it twice, but they did. Anyway they then wanted to convert it to the range 0-1 like mat2gray() or im2double() would do but they decided to do it manually. The called max() twice because the first call to max() returns the maxes of the columns, not the whole image. Doing it twice gets the overall max. It appears to be the code of someone with not much more experience than you. You can do it like most people would do it like this:
grayImage = imread('control_dic.tif');
im = im2double(grayImage);
which reads in the image only once, and is simpler, and thus easier to understand, maintain, and share with others.