Case: You have ArcGIS Online - hosted feature layer(s) or feature service(s) and you want to get the length or a summary of lengths for certain features. You don't have a map service published but need to know total lengths. Examples - Roads or routes, electrical lines, pipes or pipelines, etc. If you don't have ArcGIS for Server you will not have a map service created during the publishing stage. I don't recall if a hosted tiles layer on ArcGIS Online would carry these values, but in order to tile data you will use much more storage. Length seems like a very simple and core part of spatial features that should be accessible.
This idea has been merged with Calculate geometry in AGOL to find the area or length of a polygon or line.