Comment by biercarsten

I agree. Many people can hardly tell the difference between Arial and Helvetica. There used to be a website where you could test how good you are at telling them apart.

You won't notice many small differences between certain fonts. But that doesn't mean they're unnecessary. As you said, they make a difference when taken together. For screens, there are a number of adjustments and techniques that improve screen readability. Hinting, separate designs, contrast for low dpi and subpixel rendering compatibility, for example. At least some of the optimizations don't work out of the box, but have to be adjusted by designers. That's why it can happen that a font you bought for print media now requires an extra license for websites and apps.

There are plenty of wonderfully readable fonts for the web and apps that are free and sufficient for most projects. If you want something special, I don't think it's wrong to pay for it. Personally, I would prefer more reasonable prices, though.