In today’s world of creating, curating, and publishing web-based apps (computer and/or mobile), Web-based formats (typically feature services) are becoming the preferred format.
Feature Services
“The” Web GIS format that are “the layers” one searches for on the web and then use in Web Maps (and can also be used in Pro) for creating maps, which then can be used in web apps (e.g. viewer apps, editor apps, dashboards, experience builders, and/or mobile applications (Field Mapper).
See the Feature Layer below that shows in the Content area of the user’s content. Notice also items such as web maps, experience builder, and not shown, web applications (aka web map apps/web apps) and dashboards. NOTE: AGOL can also be used as a “storage area” for other kinds of data (or files) such as zipped up shapefiles, CSV, file geodatabases, pictures, raster imagery, and documents such as PDF, etc. PDFs and other image (e.g. jpg, png, gif) if an app needs to refer to a “document” or you need to access a unique icon for styling.

Image Services
These represent the aerial image sets, UAV image sets, satellite image sets that are often found in web apps.
Other Services
Geocoding
Provides the ability to “find” a location by address, cross street, point of interest, parcel, etc.
Network
Provides the ability to “route” (point to point, mutli-point) and are the basis for “service areas,” least cost path, optimized routing, etc.