People can learn languages without a grammar lesson. I think that’s a fact that can be accepted at face value. After all, how do you expect two-year olds pick up a spoken language? They don’t exactly attend class, do they?
Even as adults, though, learning a language doesn’t require wrapping your head around the rules of grammar. Simple activities such as memorizing vocabulary, practicing common phrases with a group or getting into an exchange with a native speaker can be all the learning that a second language student will require.
However, there are people who feel that they need to “understand grammar” before being able to grow into a new language. Some folks just have that personal barrier, where they need to learn proper form before being able to sufficiently learn a new way of ロゼッタストーン speaking. For these folks, studying a book on grammar along with their primary learning material (such as a comprehensive language training program) will serve them very well.
On the other side of the coin, there are many individuals who feel threatened by studying grammar. I know this first-hand because I am among them. While I do have a good grasp of the rules of English, studying it from scratch on another language just daunt me. Luckily for us, language learning can proceed steadily even without it.
So, to answer the question: you don’t need to learn grammar to gain facility in a new language. However, if you prefer its structure over the looseness of other activities, it might serve you well too.