Image conversion and resizing for the web can be quite fiddly. Take responsive websites for instance. You want to show a smaller version of your image on smaller devices: You don't need to download a 1080px wide image to show on a 360px wide device, especially since that device is likely constrained in the amount of bandwidth it has. Then again when your website is shown on an ultra HD screen with 3840x2160 resolution 1080 is maybe even too small.

Apart from multiple sizes, you also want to offer multiple formats. Modern browsers support new(er) image formats like webp and avif offer better compression for comparable image quality. Using these formats you can decrease the total download size of your page, while improving the overall experience for the user. But you just can't assume (yet) that all browsers support those newer formats, so you have to provide older formats as a fallback option. All in all the amount of different files you have to offer for just a single image on a web page starts to become quite large and the whole thing becomes, like I said, fiddly.