69 votested commented
Without this functionality, many users need a separate network drive (back to the old home drive) to store large work files that are not actively needed. Syncing them to local drive would fill the disk. So either synced folders should be selectible or folders/files marked as online only (until opened). Dropbox is great for looking at how this can be done properly.
I should add that I work in large environments that cannot use OneDrive or any external cloud service for this due to data classification and partly by country laws. Work Folders would be the most ideal of the remaining options.