How mean and deviation come out with MNIST dataset?
mean : It is the mean of all pixel values in the dataset ( 60000 × 28 × 28 ). This mean is calculated over the whole dataset.
deviation : It is the standard deviation of all pixel values. The dataset is treated as a population rather than a sample.



  What are the uses of these values?


Mean and standard deviation are commonly used to standardize the data in this case the images. Standardized data has mean close to 0 and standard deviation close to 1. You can read more here.


  Why to standardize the data?


Standardization transforms your data in such a manner that it has unit variance.
According to Wikipedia,


  In statistics, the standard score is the signed number of standard
  deviations by which the value of an observation or data point is above
  the mean value of what is being observed or measured