[Edit: Seems like the -u flag is on debian systems such as Ubuntu. Did a man on a FreeBSD system and saw it didn’t have the flag available.]
I use the cp and mv a lot that the -r and -f flags almost always accompany them. I also now realize I’ve used them for so long with so little thought that I forgot that there were other flags. My newest infatuation is the -u flag.

Basically -u lets you do a merge, which came in handy when I had to manually update wordpress for the first time. I could tell you more or you could RTFM