A website isn't helpful if we can't read the text. Users need to be able to enlarge the text up for web and mobile applications to be accessible. The Web Content Accessibility Guideline (WCAG) success criteria states users should be able to zoom text up to 200% without any assistive technology (except for video captions (part of a video) or images of text (this is one reason images with text are NOT accessible). When the text size is changed, the rest of the page or app keeps all of its functionality.
Relates to: WCAG 1.4.4 Resize Text (Level AA)