Sometimes looking at Analytics charts for a folder is not good enough, we'd like to slice and dice with custom fields as well.
Use case: we use custom fields to categorize tasks (business area (B2B, B2C...), type of task (maintainance, dev...), ...). Since we need to look at different combinations, doing that with folders would mean a lot of folders (cartesian product), plus we'd have problems with filtering (see http://vote.wrike.com/forums/125507-general/suggestions/2201383-add-the-ability-to-filter-tasks-by-multiple-folder and https://help.wrike.com/hc/en-us/community/posts/115000116509-Filter-Tasks-being-in-multiple-Folders-having-specified-Tags), making dashboards etc. So we opted for custom fields. But - we can't slice Analytics charts using these fields.