Right-click, Save image as, works for a single picture, but it is slow when a page has dozens of images or when the image you want is a lazy-loaded background rather than a normal file. An image downloader extension scans the whole page at once and lets you pick which images to keep.
Downloading all images on a page at once
A good image downloader scans the current page for every image element, including ones that only load as you scroll, and shows them as thumbnails. You can then select all or a subset and save them together, often into a single folder named after the page or site.
Filtering by image size and file type
Pages often include small icons, logos and tracking pixels mixed in with the images you actually want. Filtering by minimum width or height removes that clutter, and filtering by file type such as JPG, PNG or WebP lets you skip formats you don't need before you download anything.
Saving a single image without an extension
For one image, Chrome's built-in Save image as option from the right-click menu is enough and needs no extension. An image downloader earns its place when you need many images from one page, want to filter them first, or need to capture background images that a simple right-click cannot reach.