ARIA can provide things like accessible Name/State/Role/Value where it is not provided with custom controls. I think a better place to start would be understanding the requirements of Section 508 and WCAG.
508 is a requirement for the Federal Govt. but WCAG is what most of the private sector is tested against. These standards cover a lot more with respect to accessibility than just what is provided by ARIA. ARIA won't provide things like keyboard functionality but does help ensure that content is conveyed properly.
https://www.w3.org/TR/UNDERSTANDING-WCAG20/Overview.html#con...
508 is a requirement for the Federal Govt. but WCAG is what most of the private sector is tested against. These standards cover a lot more with respect to accessibility than just what is provided by ARIA. ARIA won't provide things like keyboard functionality but does help ensure that content is conveyed properly.