There are many discussions about andragogy. Some people relate it to adult learning and some other relate it to adult self learning. This comment doesn't pretend to clarify that, but to support the idea that teaching and adult is very, very different from teaching children and so it works for learning.
When a child is in an English speaking country, he gets the language through exposure, trials and correction. Basic Grammar is included without rules or explanations. It’s just like that! Learning English along their life experience, children quickly integrate it to their understanding and expressing their communicative needs. They learn from the usage and only later they study the grammar and structure of the language. The basics have already been acquired.
This “Natural approach” to English language learning gets excellent learning output, undoubtedly. The problem is that it requires long periods of time, totally immersed in an atmosphere that uses English language for communicative purposes.
When teaching adults, conditions change a lot. Adults already have their own mother tongue strongly rooted in their ways of thinking, expressing and organizing the sentences to communicate. They’ve got it after many years being exposed to their mother tongue and culture. Just this fact forces things to be different.
Apart from that, adult people taking an English Course, don’t usually have the “exposure” time to acquire it as a regular lifestyle skill. So, it takes longer periods of classes focused on practices among students in group work activities, propitiating opportunities to exercise and strengthen up their communicative skills.
Adults learn better from the reason to the practice. It’s like checking up the rules of a game before playing it. Adults will feel better, perform better and can be more proactive when they know where they pretend to go or what they pretend to reach in a class activity.