You rediscovered normal saline!

The plain water hurts because it's causing cells to swell or even burst as water rushes into them to equalize the osmotic pressure. Adding a little bit of salt to that helps remove that pressure because the environment inside and outside of the cells are both equally salt.

Well, not so much rediscovered it as reasoned that that might be the cause of the pain and then confirmed it. The shocking part is that no doctor advises to use this after a tonsillectomy (or other kind of mouth or throat procedure).

My dad (also a doctor) called it a typical case of "the nurse's wisdom": the kind of quality of life interventions that typiqally get discovered by nurses and passed down orally, but never make it to official medical journals.