Currently few projects are developed following the waterfall model. And perhaps even those projects don't have 100% of its database tables set up before development begins. That's because software development is dynamic... and almost always the software changes during the project.

What we must know is that we can not start a new project, generate all the initial code and finalize the project with a final customization.
This type of situation does not happen in practice. We may have to start the project developing some features, and then we will have another list of features to get implemented.
We do not need tools for generating code only for the start of the project. It should be used several times in the development of the project, creating new artifacts, forms and CRUDs if necessary.

So an incremental build that creates some artifacts now, some later, and manages to join all them in a project without disturbing the code is an important factor of motivation for the use of the tool.

Another detail is that we are not forcing, nor worried if you want or not to generate the entire project using SPIDER. You may choose what you want to do, the idea is being free. If you want to generate only the project's framework settings (xml files and libs), and get to work, okay. But if you want to also generate packages and modules, so can you. If at any time you want to generate a CRUD, will not have problems too. The generation has not fixed characteristics. You are free to include whatever you want in your build and continue development from that point.