Web-based editing allows a larger group of people to add and improve on your data. These include field-workers, analysts in other departments of your company, or perhaps even crowd-sourced volunteers who may be able to make valuable contributions to your data given a simple editing interface.
The role of the feature service
Web editing works with features, which originate from feature services (in ArcGIS Server) or feature layers (in ArcGIS.com or Portal for ArcGIS). You can define feature templates in the map, which represent predefined feature types that might be most often created by editors. For example, to prepare for editing roads, you might configure templates called Controlled Access Freeway, Other Divided Highway, State Highway, and Local Road. Users of your application can then select from one of the template choices instead of configuring the attributes themselves. This results in a simpler editing experience and less opportunity for error.
The role of the geometry service
The geometry service is also used to support editing. The geometry service can create, cut, and reshape features. It can also perform integrity checks, such as making sure polygon boundaries do not cross themselves.