Imager Transforms, Cacheing and S3
Imager will always create and store the transformed images locally in your imagerSystemPath, even if you choose to upload and serve them from an external storage. The files in that folder acts as the cache, so that Imager knows if a transform has been created or not. 

Alternatively, Imager either would have to; 1) make a request for each transformed image to see if it existed in the external storage, or 2) have some other method of keeping track of what transforms has been made (DB, DB-like file, or something). Option 1) adds an insane amount of overhead, whereas 2) adds quite a bit of complexity and possibly some overhead (like Craft's built-in transforms, which requires DB queries). So I opted for speed and simplicity, and sacrificed disk space. :)