150 years ago communication between unmarried persons of the opposite sex was closely monitored for any hint of impropriety.
So say it with flowers was greatly expanded to a vast repertoire of hidden messages... except to everyone who knew the "code".
I suspect most young ladies did. The guys would probably have to ask a sister or look it up if he could even figure out what kind of flower it was. Kind of inconvenient and impossible to send a dick pic or nipples.

Some of these are rather dark, I wonder how often they updated.