Unless your double spread occurs on the two center-most pages of your comic, then your double spread gets split, printed on two separate sheets of paper, and the binding simulates the look of a spread by then juxtaposing the two pages next to each other. Because of those steps we need a margin for error. Hence the center margin area in the double spread template. If you place word balloons and text in that center margin area then those word balloons are going to be split, printed on two separate pages, and very likely not lineup properly when reassembled in the binding which will make them largely unreadable or at the least difficult to read.
Similarly, you should avoid putting any crucial element of artwork in the center margin area. For example, if you have a character gesturing or pointing or if you are depicting a certain facial expression on a character or revealing any visual of import to the scene or the story then you don’t want to risk that it might be unclear or distorted if the two pages don’t align precisely in the binding.
What’s more if the spreads are ever printed in a square bound edition any word balloons or crucial artwork in the center of the spread will be lost in the spine. It’s the simple physics of square-binding. When a squarebound book is opened there is at least an eighth and maybe as much as a quarter an inch of the spine side of each page that the reader simply can’t see because it curves into the binding.