It’s a whole thing. Instead of refreshing a cup throughout the day or getting new beverages as you’re thirsty, you simply fill up one cup at the beginning of the day and carry it around everywhere you go. Some people do it because they have a hyper specific (read: hyper branded) preferred beverage. Some people do it because they read an article that told them that if they didn’t drink 12 gallons of water a day that their asshole would fall out (they don’t trust their kidneys). Some people carry around several cups: one for caffeination, one for hydration, one for taste, etc. A lot of people just carry their caffeinated coffee milkshakes around.