Image resizing adjusts an image's actual pixel dimensions to match the size it will display at, rather than serving a much larger original and letting HTML or CSS scale it down visually. A 1000x1200 image displayed in a 400x500 space still forces the browser to download the full, larger file, even though the visitor never sees more than a quarter of that resolution rendered on screen.
That gap adds up fast on pages with many images: a product listing grid showing forty thumbnails at 300 pixels wide, each secretly serving a 2000-pixel original, can add megabytes of unnecessary download weight for zero visible benefit. Responsive images, using srcset to offer several sizes and letting the browser pick the right one for the visitor's actual screen, handle this properly across devices without needing a separate manually-resized file for every context. Resizing and compression solve related but different problems, resizing shrinks pixel dimensions, compression shrinks file size at a given dimension, and both are worth doing together for the biggest speed improvement.
