They're relatively inexpensive
Memes are created by other people, so you don't have to create an original photo yourself. For example, I use IMG Flip to look for a particular meme and use it to create a new caption.
They quickly go viral
Memes are meant to be viral, so people will instantly share them. The more your content is shared, the more traffic you get.
They can be used to attune your brand
The popular guy-checking-out-girl meme, in particular, is all over the Internet that even brands are onto this, including Ruffles:
So what's the next meme you're sharing on Facebook? What's your favourite meme?