Why should we teach grammar to L2 learners?

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Aside from the first comment, grammar actually exposes students to:

• structures of L2 found 'irregular' in grammar, yet 'regular' in everyday speech (e.g. ES La gente está lejos. vs La gente están lejos.);

• using declensed words where they would normally use them less and not gain as much practice (e.g. genitive in RO in compound nouns like floarea soarelui);

• overall, getting an insight on L2's grammatical structure thus enabling autonomous behaviour in L2 (e.g. student learnt IT verb prendere in school and in real life they knew the past participle of comprendere was compreso by analogy);

• an older language's grammar, thus enabling the student to deduce how the word would behave in a modern language (e.g. LAT -tas, -tatis gave ES -tad/-dad, PT -tade/-dade, RO -tate, IT -tà, FR té etc.).

Grammar shouldn't be the first thing we teach in L2 classes, but it needs to be taught at some point. Grammar is all about patterns. Some students will pick up on these patterns without explicit instruction, but most won't, especially if the patterns are very different from those in the students' L1. Explaining these patterns to the students makes it easier for them to remember what morphemes should be used where and when.