When it aligns with their preferred narrative.

Anecdotal isn't even used right these days.

"My uncle got sick when he ate at that restaurant." This is an anecdote.

"I got sick when I ate at that restaurant." This is a direct observation of correlation.

If someone else uses my direct observation alone, it becomes anecdotal. If someone collects similar observations from 20 different people, it becomes evidence that something is wrong at that restaurant.