The first round of cooking mushrooms removes any excess liquid from the mushrooms. This liquid can be drained and saved for other recipes or it can be reduced until the liquid is gone. The second round of cooking sears the mushrooms so they get a golden brown color. You can also add additional flavors like garlic and butter during the second saute. You'll end up with a delicious concentrated umami flavor and a wonderfully meaty texture without any sogginess.