Kup is created for helping people to keep up-to-date backups of their personal files. Connecting a USB hard drive is the primary supported way to store files, but saving files to a server over a network connection is also possible for advanced users.
When you plug in your external hard drive Kup will automatically start copying your latest changes, but of course it will only do so if you have been active on your computer for some number of hours since the last time you took a backup (and it can of course ask you first, before copying anything). In general Kup tries to not disturb you needlessly.