Visual information required to identify user interface components (like the border or background of a text input field), and what state they’re in (like focused, or in error), must have a 3:1 contrast ratio against adjacent colors.
if the appearance of the component is determined by the Operating System/Web Browser, and not modified
A text input field doesn’t have border or background colour with enough contrast, so it’s hard to see or understand that there is a text input.
A text input’s focus indicator doesn’t have enough contrast, so it’s hard for users to notice that the text input is selected.
Example: Link represented by an icon
For graphical objects (like icons, or parts of an infographics)
Any visual element that users need to see in order to understand the content of a page must have a 3:1 contrast ratio against adjacent colors.
The purpose and meaning of important images - like if a medicine is poisonous, or other warnings - are hard to see for a user who is sight impaired.
Example: star rating
Many sight impaired users cannot see important controls or understand graphics if they have poor colour contrast.
Official wording in the Web Content Accessibility Guidelines
User Interface Components: Visual information required to identify user interface components and states, except for inactive components or where the appearance of the component is determined by the user agent and not modified by the author;
Graphical Objects: Parts of graphics required to understand the content, except when a particular presentation of graphics is essential to the information being conveyed.