Why multiple copies of media instead of simply resizing?

I’m wondering whats the advantage of creating multiple copies of images at all the various sizes rather than just having one copy (the largest) that gets resized using the width and height attributes.

I”m trying to decide whether to add a new media size via functions.php (600px) or to just keep doing what I’ve been doing which is using the largest image, manually doing the math to figure out the new height to match being 600px wide and then changing the image attributes. I would definitely like a more streamlined way and adding a new image size (or redefininf “Medium” size) seems the way to go but it just seems like a waste of space to upload an image and then have WordPress create several copies of it at different sizes.

Thanks!