This feature allows you to create a backup copy where only those files will be included which have been changed since the last backup. If your blog/files have a considerable size (eg. 1GB+) and you are used to backup your files regularly on daily basis then usually (see full backup above) you must (i) compress all those files thus blocking your system resources and (ii) to transport the resulted backup set to a remote storage location which means using a huge amount of your network bandwidth. With incremental backup you just compress and transport those files changed since the last backup thus it (i) finishes earlier and (ii) uses less system resources than with the normal full backup (eg. on a blog with 10GB+ files if only two files are changed since the last backup then the backup will finish instantaneous!).
As for the backup the incremental backup mode is faster than the Differential backup which is faster than the full backup. As far as it concerns restoring the complexity is inversely proportional.
On the other hand this backup type is harder to restore because you have to restore each incremental set at a time, in their creation order. If you have the Restore Wizard then restoring an incremental backup set is almost as easy as drinking a glass of water.