What “modular” means in a closet
A modular closet is assembled from a defined family of widths, towers, shelves, rods and accessories. The advantage is predictability: components are designed to work together and the system can often be planned quickly. Some modular systems also make later changes easier.
The trade-off is that the room must accept those module dimensions. A leftover strip of wall may be unavoidable, or a drawer tower may not align with a reach-in door opening.