Repetition is a necessary part of language learning. It therefore must be a part of language teaching. Repetition on the side of the teacher, however, is often inefficient. A student may need to hear the same material 3, 4, 5 or more times before it sinks in and makes sense. Relying on a teacher to provide this repetition is, in my view, often a waste of a valuable resource.
Years ago, I was skeptical about the use of video in education. I was wed to the principle that the interaction with a teacher was irreplaceable. And it is — but I've come to realize that one typically doesn't get that interaction when the teacher has to lecture and repeat core material over and over again.
In my experience as a learner, digital forms of repetition are fantastic. I can get instant repetition without having to wait until the next session with a teacher. I also don't have to worry about annoying the teacher by asking the same questions again and again.
In my experience as a teacher, providing students with a digital means of repeating core material is likewise fantastic. It means they can get the repetition they need without drawing on my time so that when we do have sessions together, we can get the most out of that time. It means that the interaction between student and teacher is more engaging and focused on answering questions, applying knowledge, and practicing core material in context.
Knowing what can and cannot be committed to a digital form poses some issues in developing online language learning programs. Core materials should be committed, but it is a lot of work to develop those materials. Furthermore, one does not benefit from the natural variation of a teacher's lessons when delivered in person. If a certain degree of variation of accent and register can be introduced in digital form, this enhances the language training program considerably.