Start with the difference in design logic
A modular product line asks, “Which available modules fit this wall?” A custom approach asks, “What does this wall need to store, and what dimensions work best here?” Both can produce an organized closet, but the sequence of decisions is different.
If standard modules fit a straightforward room well, modular planning can be efficient. If the wall width, doorway or desired drawer location falls between module sizes, custom fabrication can use the space more precisely.