There are different types of backup systems.
File-based backup systems operate "above the filesystem" (this includes things like SyncBack, GoodSync, and Dropbox). They can detect changes, and only copy changed files, but when 1 byte of a 6GB file changes, the entire file must be copied over again. The "atom" is the file. The file is either changed or unchanged. These have benefits, though, as they can perform changes "live" on the filesystem much more easily, and can sync to dissimilar filesystems. I use GoodSync to sync my documents folder, among others, from my Windows machines at home to my Macs at the office, for example.
Block-based backup/imaging systems (like Norton Ghost, Acronis True Image, and Macrium Reflect) operate underneath the filesystem, on the raw data on disk. The atom is the disk logical block. They don't know or care what "files" exist in the filesystem, they only see the actual blocks and sectors on the disk storage system. This allows all sorts of fancy things, including being able to clone parts of the disk that aren't normally user-accessible, and when 1 byte of a 6GB file changes, the imaging system (which doesn't even know the 6GB file exists) can sync only the block where the 1 byte changed physically. They have downsides as well, though, as you can't easily clone from a NTFS to a HFS+ (or FAT32) volume this way, and mostly they are designed to make full "images" of the partition you are backing up. It is much more challenging for them to back up just a particular folder, or set of files. They operate on entire volumes (though Macrium has some of these features too).
Both are handy. What tool is right for you in a particular situation depends on your needs. I use both types of systems for different purposes.