'Distribution' argument for fitglm() in Matlab

I am using the Matlab fitglm() function for a logistic regression based binary-classification on some data. I have a question regarding one of the optional arguments of fitgl() function. The argument 'Distribution' as mentioned here in the Matlab's fitglm() documentation, the optional argument Distribution for fitglm() specifies the distribution of the response variable. I dont understand this. Why we have to make an assumption about the distribution of response variable. I thought we can only make an assumption about the distribution of predictor variables for the two classes. What is wrong in my understanding ?