There are requirements for users to take a number of different areas offline for the same webmap at once. This could be if they are going on field work and wont be able to connect online to sync up edits and take another area offline. Due to the dispersed nature of some field studies it would be unfeasible to take entire areas of a number of study sites offline.
This idea has been marked as a duplicate of Download Several Pieces of the Same Web Map in Collector for ArcGIS by Scott Prindle. Please consider voting on the other idea.