I'm not sure.
For post-growth alerts, a message at the beginning of the turn would be nice. But, if you have many different messages, it can be inconvenient to keep track of them all, so it's nice to have the information somewhere else, too.
And, for pre-growth alerts, a message at the beginning of the turn could be completely forgotten by the time you get to the end of the turn, which is when you're more likely to be managing growth for next turn. And, there's also the possibility that you make changes during your turn that create growth when it wouldn't have existed at the beginning of the turn. In that case, a beginning-of-turn alert wouldn't be effective at all.