Success criterion 1.4.10 exists for users with low vision who rely on significant browser zoom rather than a dedicated screen magnifier. A page that reflows correctly rearranges its layout into a single readable column, similar to a responsive mobile view, instead of forcing the user to scroll sideways to read each line.
Sites already built with responsive design usually satisfy most of the reflow requirement automatically, but fixed-width tables, horizontally scrolling data grids without an accessible alternative, and rigid multi-column layouts are common places it still fails.