The Rule of Threes states, humans can survive three weeks without food, three days without water, three hours without shelter, and three minutes without oxygen.
In an emergency evacuation, hopefully oxygen won't be an issue and finding a way to shelter will be a planned-for event using a designated emergency route. Three weeks is a long time to go without food, but, even in the worst of emergencies, food should be available long before those three weeks go by.
The most likely issue that arises in an emergency situation is a lack of water, at least clean, drinkable water. Planning for this possibility is essential.
Portable water comes in many forms - bottled, packets, jugs - and though it's quite heavy, weighing in at 8.34 lbs per US gallon, it's easy to stock. Having sufficient water in your GO-bag is important, and having a way to sanitize water is a good idea as well. A vessel, ie. a bottle, that has a wide mouth, measurement lines to be able to measure out the water for purification tabs, and a tight cap is essential to being able to deal with sanitization.
Water is life. Having sufficient, potable water available in an emergency situation can be life saving.