The reason is to scale teams. It’s not the only way to do it—you can also have vertical teams—but it’s a common one because frontend and backend have different

The reason is to scale teams. It’s not the only way to do it—you can also have vertical teams—but it’s a common one because frontend and backend have different technical considerations. The downside is the product can lose cohesion as developers get tunnel vision. Of course all big teams suffer from a version of that problem depending how the lines are drawn.