There are so many reasons why I advocate for this, but the two primary ones are:
We get to know each other better and build trust before the wedding day. This lowers some barriers allowing me to capture more authentic portraits sooner in the day.
The other is that we get to learn and practice the communication system I use for posing and prompting. We get to practice and experiment with what types of prompts work best for the two of you. Allowing us to move straight into creating amazing images and skipping the learning/practicing part.
Knowing how your photographer works before the wedding is only going to help you get even more out of your final gallery.