A fly box keeps your flies organized and accessible on the water. Slotted-foam boxes are the standard: hooks slip into pre-cut slots so flies don't flop around. Magnetic boxes hold metal-bodied nymphs; compartmented boxes work for streamers and bigger patterns.
Most fly anglers carry 2–4 boxes — one each for dries, nymphs, streamers, and a "favorites" box for the working patterns. New anglers can start with one slim box of basic patterns and expand as needed.