We often say "say" when we've got no information, we just want to show that the chronology is reasonable.
WikiTree has another good reason for "say" dates - the search system can't do anything with undated profiles. This forces me to put dates in that I don't want to.
I usually flag them with a {{DateGuess}}, which I leave out when there's specific but approximate evidence.
But other people don't like the templates and remove them. They figure, if they just state the unsupported assumptions behind a "say" date, or cite somebody else's guess, it's not a guess any more.