Paying attention to students with different skill levels requires much time during onsite training. Students with more knowledge can get bored easily. Vice versa, students with less knowledge may get overwhelmed.
The flipped classroom method aligns the different skill levels before the onsite training. The students study the training material at home at their own pace. Depending on their knowledge, they need less or more time for understanding the training content. Students with less knowledge do not slow others down. Advanced students can skip material they already know and use the time for other topics or tasks to deepen their knowledge.
Luca Leonie Jäger has written a fine knowledge article about the subject: Flipped Classroom – How to Make Your Training More Efficient.