What's the meaning of "do something for nothing"?

It is means somebody do a invaluable and unnecessary things！

忙碌而一无所获 例如 You can't get something for nothing. 人不能不劳而获。

do something for nothing做一些无关紧要的事情。

一无所获

应该是说不计报酬地作些事情
比方：

I like my job so much that I will do it for nothing!

"不劳而获"