I've dl'ed them as txt and as csv files. The csv file opens with Excel. Both have their advantages but I'd recommend the csv. Mine were automatically saved with the save date in the file name.
As far as reasons for saving... some were for sentimental value and some just because it seemed wise to save them. They are history. The txt file was the largest and it was only 11MB so they don't take up much space on the HD.