The point is that the writers need to get Dany very angry so she becomes mad queen. There is no practical point in terms of the story of the characters themselves. But what is the point of going to King's Landing by boat? Because the writers need a dragon killed and Missandei captured. But you can't go on boat from Winterfell, so have them go to Dragonstone first. They do that because the writers need Dany there before the northerners arrive so that Jon can see mad Dany killing innocents. There is no practical reason why they need to go to Dragonstone instead of just marching south together.

The show is not character-driven, it's plot-driven. It goes by pre-planned, broad plot points and the writers make the characters do things to make the plot points happen.