When the images on your website are sized properly, your site will have far more SEO advantages and load much faster, making it a better experience for visitors. Ultimately, we want visitors to stay on your site and explore your content longer. If things move too slowly, people will move on to one of your competitors.
Often resizing and/or reducing image sizes will go a long way in making your website faster and greatly improve your SEO ranking (site speed is a ranking factor). As a standard of practice, we do this on all the sites we develop.
If you are working on your own website content, here are some resources you can use to help you make your images as effective as possible: