Many browsers offer the ability to zoom in and out of a page, and some even allow for the ability to set a text only zoom within the browser preferences. So why consider an on-page font-size adjuster?
Although font and page zoom adjustment is a great feature when considering the demographics of diminishing vision. Most web pages are not designed to hold their original design with the onboard browser font/zoom manipulation capabilities.
Often subtle or even extreme format shifts can take place, disrupting the integrity of the page and creating a distraction. Menus can become disjointed, lines spacing can cause overlapping, quality of images will degrade. Unfortunately even the most carefully prepared and screened pages are not designed to be manipulated in these ways, and although some pages may look fine, a majority will not.
By incorporating a practical on-page font-size adjustment component into the design of a website, testing during development can address potential undesirable format shifting, and select areas where the font size may be an issue can be targeted for adjustment. If a font adjustment is visible on the page, the user is less likely to use the onboard browser zoom.
Allowing users the ability to adjust the size of font that feels most comfortable to them, just makes sense.