First, I believe this is important mainly because low level learners spend a lot of time asking and responding to questions. The main avenue they have to produce meaningful non-question sentences is when they are responding to questions. They need to practice giving answers in complete sentences. This will help them learn the vocabulary as well. An example of this is: Do you like apples? Yes. But this would be a better answer. Do you like apples? Yes, I like apples. Furthermore, ESL students need to practice the questions.
Secondly, sentences make it easier for students to learn English because they can start to associate words and this is going to improve their vocabulary much faster than just trying to teach them individual words or phrases. Getting younger students to go beyond Yes and No answers will help them with writing and reading as well.
Thirdly, by teaching complete sentences you can start to exclude a word from the sentence to see if they know what the word is. This is a great exercise for faster learning. Speaking in complete sentences give the ESL student the chance to show the student variations in the way they can say the same thing in a completely different way.
Lastly, they need practice. I constantly ask students questions throughout the class. This is so important to make sure they understand what they are learning but I make sure they speak in complete sentences.
Teachers need to set the expectations from the first class. Always get them speaking in sentences. If they can only do short sentences then that is OK. You have be consistent and correct them all the time.
Here is a video on getting student to answer questions.
If you have any other strategies to get students to speak in complete sentences please post them below. Happy Teaching!