A seasonal campaign section gets added and never removed after the campaign ends. An app gets uninstalled but its theme snippet stays referenced. A CSS override from a one-off fix never gets cleaned up once the "real" fix ships. Each individual addition is small; the sum isn't.
Extra JavaScript and CSS slow down every single page load, not just the feature that needed it. Orphaned code becomes a liability during theme updates, since nobody remembers if it's safe to remove. New developers working in the codebase inherit confusion about what's actually load-bearing.
A quarterly audit — removing sections unused in the last publish, auditing for orphaned app snippets, and checking actual CSS/JS payload size against the previous quarter — catches this before it becomes a full theme rebuild. Small, regular cleanup beats an occasional massive one.