The philosophy of the SharePoint Framework takes many cues from past missteps of the add-in model and full-trust solution model. As developers have been moving to a more lightweight approach using script injection, embedded HTML within a script editor web part, and similar models, the SharePoint Framework aims to provide a unified and consistent way to create customizations in SharePoint.
All development for the SharePoint Framework is modern client-side development. This does not imply that nothing can be implemented on the server side. It merely means that most SharePoint customizations tend to be functionality that can be encapsulated on the client-side quite easily. For those features that require server-side logic, it can be implemented outside SharePoint; typically using a serverless platform such as Microsoft Azure Functions, Azure API apps, or similar concepts.
The modern approach also means that the SharePoint Framework will evolve over...