Yes, Developers can structure the projects in whatever way they want, but the structure that we have given is the recommended approach. SaaS applications are always a bunch of modules and its related sub features, so it is always better to structure the solution in the prescribed approach.
Below are some of the benefits of the prescribed solution structure, they are
- Code Separation
Atomicity - Isolating the respective DAL(Data access Layer), Service Layer of each individual Modules
- Unit Test-ability
In specific cases, where you want to have a separate project/solution to deal just with DAL or business layer, you can definitely have.