Suppose data is sorted in a specified order. For example a data set which is sorted base on their class. So, if you select data for training, validation, and test without considering this subject, you will select each class for different tasks, and it will fail the process.
Hence, to impede these kind of problems, a simple solution is shuffling the data to get different sets of training, validation, and test data.
About the mini-batch, answers to this post can be a solution to your question.