My essential is a small crossbody that rountinely holds my iPad and any miscellaneous items (program, guidebooks, etc). When flying or on embarkation day it also holds a charger (for iPad and phone), small bag with 6 Tylenol and 6 Tums and a couple bandaids. Plus a small packing cube with a spare pair of underwear, ankle socks, jogging shorts, t-shirt, toothbrush, toothpaste, comb, and deodorant go in it.
You only have to have the airline delay your bags once to understand the need. Or get caught in the rain and be soaked on embarkation day, or spill a coffee on yourself, or whatever. Being able to put on dry or clean clothes rather being miserable waiting for your luggage is a good thing.