Mindfulness simply accepts whatever is there. If you want to grow in mindfulness, patient acceptance is the only route. The process cannot be forced and it cannot be rushed. It proceeds at its own pace. Mindfulness directs the power of concentration.
Mindfulness is the manager of the operation.
Concentration furnishes the power by which mindfulness can penetrate into the deepest level of the mind. Their cooperation results in insight and understanding. These must be cultivated together in a balanced ratio. Just a bit more emphasis is given to mindfulness because mindfulness is the center of meditation. The deepest levels of concentration are not really needed to do the job of liberation.