One of the drawbacks of the listen-and-repeat method is that you learn sentence patterns without explicitly learning how the patterns have been constructed. You may, for example, learn these sentences:
What is this? It is a pencil.
What are these? These are pencils.
What is this? It is a book.
What are these? These are books
You are pretty much left on your own to figure out the rule that an “s” at the end of a word signifies plural, and why the plural of “this” is “these.”
This approach does work, and has the advantage that you will figure out a rule on your own. It is your rule, you own it, and you’re unlikely to forget it.
The problem is that certain rules are tricky to discover. Russian singular nouns, for instance, have five different endings depending on how they’re used in a sentence. Determining the rules for adjectives in Russian is a nightmare. However, a grammar book can tell you what the rules are and reduce three sets of five endings to one set and two simple spelling rules.
That’s what grammar provides you with—the set of rules that assists you in figuring out why the sentences you’ve learned to repeat work the way they do.