This is a powerful tool! Use at own risk! Nobody is perfect and Android neither. Apps you have already deleted, sometimes leave data behind. The system constantly creates logs, crash reports and debug files you don't really need. Your SD-card is collecting files and directories you don't recognize. Lets not go on here...
- SD Maid is for storage optimization an not for phone optimization
- SD Maid helps with storage space, while greenify is all about battery optimization
- SD Maid a cleanup tool and manage installed user and system apps, while Greenify is a hibernation tool.