The Additional Accessibility Features plugin does not solve any accessibility problem your site can have. The website needs to be built correctly for accessibility from the start.
People with disabilities use specialized tools (screen readers, screen magnifiers, braille embossers, etc.). and will probably not use the plugin, but it can improve the usability for
the elderly (e.g. text enlargement: users often do not know about the possibilities of the browser and like to use such tools)
people with visual impairments (e.g. invert colours)
people with cognitive problems (e.g. reading guide)
people with reading issues (e.g. text to speech)
WCAG does not require the links to be underlined (they must be highlighted). But underlined links can improve accessibility for visually impaired people or for people who have difficulty concentrating.