Screen magnifiers are used by people who have low vision. It enlarges the text and images to a size where only part of the page or picture is visible at any one time. It is not the same as using the zoom function in the browser where the layout is changed to match the width of the screen. This means users have to scroll sideways to get the content of the page.
Because only a portion of the screen is displayed, the reader could miss instructions or drop down menus. How to Make Your Website Accessible to People Who Use a Screen Magnifier, has some good tips and a short video that shows what a screen magnifier does. The tips in the article are aimed at people in charge of websites. However, it is useful to see how it works and what happens when a site design doesn’t account for screen readers. It could help us think about the way we format documents and other information for websites.
It’s worth noting that there are around ten times as many people who use magnifiers and not screen readers, but accessibility guidelines seem to focus on readers. There is more on this topic from Axess Lab.