The “box filter effect” is only really helpful to someone who understands that the appearance of the magnified boxes is not remotely like the appearance of the final image. Its main utility is showing pixel artist that the samples are discrete, and very explicitly indicating their spacing.
I don’t agree with you that the upsized box UI is necessarily the most intuitive or useful, but instead is just the only UI that has really been tested. I can imagine several other possibilities, for example showing a smooth interpolation (indicating something closer to real-world appearance) overlaid by a circle at each sample center (indicating color of the discrete sample). [That's an idea off the top of my head; I don't know if it would work in practice.]
I don’t agree with you that the upsized box UI is necessarily the most intuitive or useful, but instead is just the only UI that has really been tested. I can imagine several other possibilities, for example showing a smooth interpolation (indicating something closer to real-world appearance) overlaid by a circle at each sample center (indicating color of the discrete sample). [That's an idea off the top of my head; I don't know if it would work in practice.]