Drawing Layers and Scenarios both help you organize your projects. However, they work differently and are used for different purposes.
Drawing Layers:
Geometries can only live on one drawing layer
Drawing layers control stacking - organizing built form relationships
Drawing layers can have an opacity in addition to visible/invisible status
Drawing layers are required - Every project must have at least one drawing layer
Scenarios:
Geometries can be assigned to multiple scenarios
Scenarios organize conceptual relationships
Scenarios are either on or off - no opacity setting
Scenarios are optional. Projects function perfectly with only the Base Model and no scenarios created