Is it faster for you to get somewhere by car when you……
a) Start at your house, knowing your destination and thereby ploting the quickest avalable route, or
b) Start 1/3 of the way to your destination, but you do not know your current exact location, and have been driving for 2 hours or so and may need gas shortly.
If you start in port, you know where you are, and where you are going, and the quickes route there. Also you are fully fueled and well rested. You have also traveled the seas surronding your port many times before and are familiar with the safest/quickest/most efficent route to open seas
Orderd to move at sea requires you first recieve your orders via coded message, then to take bearings, plot a course on the fly, inform other ships in the convoy of their orders, and set sail.
Perhaps some ships are already low on fuel/food/or damaged etc.