The jimu-arcgis package contains components, interfaces, classes, properties and methods for working with the ArcGIS Maps SDK for JavaScript.
When working with maps, you will regularly use the Jimu class and the Jimu component to access and use the Map or Scene instance from the ArcGIS Maps SDK for JavaScript.
Your workflow may also require other parts of the package. For example, you will need to leverage the Jimu class and the Jimu component when you must await the Layer presence to complete a task such as populating a Dropdown list of Drop of the layer's features.
In summary, if your widget needs to make use of the map in some way, you will use the jimu-arcgis package.