Most people know you should diversify your sources of income to protect from loss, but it's also a way to make you more productive.
Consider this: How many times have you come across a lucrative keyword that you know something about, but you've had to struggle to come up with a "how to" type article for that subject?
Now just imagine if you didn't bother with that struggle. You just went straight ahead and wrote the best article for that keyword that you can. Maybe it's an "about" article, or an opinion piece, or maybe it's one of those borderline articles: "Seven Ways to Do Something-or-Other" which you could force into how to format, but then you have to worry about being deleted in a sweep.
What if, in the time you normally would have sweated and flustered over how to turn an 'about' article into a 'how to' article, you just wrote the 'about' article and were done and ready to go on to something else? Or what if you wrote that "about" article, and that opinion piece, and the "Seven ways", AND wrote a how to? What if you ended up with four articles rather than one?
You can do that if you write for more than one site. (And they can all provide backlinks to others.)