Stuff like this doesn't mess with websites that much as it's all rectangles anyway. And as the comment above said you get to alternate between the sizes/proportions at will so this becomes a non issue. Use smaller screen for common stuff and then the bigger one for movies, games, comics, ebooks, displaying recipes in big text to see from far away, and whatever else you want. People could simply carry around a tabled at all times but there's the extra convenience of it fitting in your pocket and being the same device as you wouldn't have to sync data between them. This is something similar to an argument for cameras in phones, you always have it with you ready when a need comes up and a camera you don't have with you when you need it is useless.

Yes that is a good point I noticed all the time when using a foldable. Most uses are just optimized for the standard format. Also phone holders and such.