Do you focus only on one thing?

People say that you should always focus on one thing if you are doing something, like one niche, one programming language, one thing at a time at all times. It will give you better results, like if you are writing about travel, you should write only that which will make people read and connect with you more.

But for me it does not mean actually make sense. You should focus on one thing that is true but that does not mean you should ignore other things altogether. Taking the above example itself if you are writing about travel, that does not mean you cannot write about a natural calamity which was happened. Or if you are learning javascript which does not mean that you cannot learn SQL, even though you are focussing only on frontend technology.

Focus does not mean that you cannot check out other things or you cannot work on other things. I am a .Net Developer but still i work on Node.js sometimes does not mean i am changing the focus from .Net but something is better in Node.js.

What do you think about it?