[tzot]

> but that's not what happens in nature.

Exactly. When it rains in nature, 95% of the times a) there isn't enough sunlight for the droplets to focus and make a burn spot, and b) the droplets don't stay on the leaf but flow down instead.

The original advice is solid and not an urban legend, but it applies to cases like watering plants in your balcony when the sun is out, bright and hot. Source: I have caused burn spots in plants of my own.

[Kerrick]

Those “burn spots” are almost certainly from a fungal disease, not from some magnifying glass effect. https://s3.wp.wsu.edu/uploads/sites/403/2015/03/leaf-scorch....