According to the Canadian Modern Language Review, formulaic sequences in linguistics refer to a given combination of words, within a sentence, that can help in establishing language fluency for language learners, both first and second language learners. This fluency is established by having the learner say the sentences with shorter and less frequent pauses and longer stretch of actual speech between these pauses. I believe learning formulaic sequences at an early age is essential in language learning for two reasons.
First, learning formulaic sequences allows students to learn how to formulate and pronounce idioms. In English, idioms are very important. They are basically phrases composed of multiple words tied together, with a meaning that is different from the definition of each single word alone. Idioms are everywhere in the English language and mastering them is the first building block in learning any language in fact. By not pausing a lot and for too long, learners are forced to tie words together often and be able to speak them in one run. This forces the learners to choose words that tie well together and hence be more knowledgeable of their meanings. As well, in formulaic sequences, the learner gets to understand how different words, verbs and nouns tie together adequately to make a meaningful sentence. Each word in the sentence has to make sense, and if not, it will be evident when pronouncing that sentence without pauses, as in formulaic sequences exercises.
The second reason why formulaic sequences are important, is that the learner gets to actually practice vocabulary. Having to pronounce sentences in one run and without stops means the learner has to stay several words at once, and those words need to be meaningful. Therefore, the learner would need to know the meanings of each single word alone. Hence, the learner will have to learn a lot of vocabulary.
On the other hand, if the learners were to pause a lot and for long periods of time, they will find no motivation to formulate sentences and hence no need to learn how to tie words together. This defeats the purpose of learning a language. It is also interesting to note that through learning formulaic sequences, the learner is in fact, learning the language grammar, indirectly. Grammar after all is a set of rules and guidelines that govern the formulation of sentences and clauses. What is a better exercise to learn those rules without having to say multiple words together meaningfully?
As such, learning formulaic sequences and vocabulary is far more important than just learning the grammar and rules, when learning a second language!