How does the concept of a Service Layer complement traditional MVC by providing an additional place for complex business logic that spans multiple models?

A Service Layer introduces dedicated service classes that sit between controllers and models, handling complex business operations that might involve coordinating multiple models together, external API calls, or transactional logic, which helps prevent controllers from becoming overloaded with orchestration logic while also keeping individual models focused on their own specific data and behavior, rather than each model needing to know about and coordinate with several other unrelated models directly.

class OrderService {
    public function createOrder(array $data): Order {
        DB::transaction(function () use ($data, &$order) {
            $order = Order::create($data);
            $this->inventoryService->reserveStock($order);
            $this->paymentService->charge($order);
        });
        return $order;
    }
}