Beginners stand squarely facing the court they are trying to serve into:
They need an eastern grip so their racquet face will face into the court they are trying to serve into.
Advanced servers serve with the side of their body facing the court they want to serve into.
They want to launch the side
of their body into the court, using their whole body like a giant muscle to power the serve.
In order to do this, they have to use a continental grip so the racquet faces into the court as they launch the side of their body into the court on the serve.