You mean because you don't see on canvas exactly the same size?
Canvas aspect ratio, mainly.
I know graphic designers who have a great sense for composition and typography - and for this very reason, they stick to print design. They totally loathe that their composition after handoff would get scaled and pushed around, based on rules.
I know what they mean, but I'm more pragmatic. Webstudio could have great appeal to artsy pixel-pushers. Wysiwyg previews were very helpful, to make sure that the initial screens and important sections look great – at least on most screens.