Most of our ailments result from our inability to handle situations resulting in stress. Stress is speeded up mind with repeated thoughts. We understand that we should be calm under all circumstances, but it is easier said than done.
As Arjuna asks Krishna in Gita - "atha kena prayuktoyam paapam charati purushah | anichchan apai vaarshneya balaadiva niyojitah". Even if they don't want to, people sin, as if by force (we should take the word sin as anything that harms us or others).
With repeated speeded up mind and stress, we are causing harm to ourselves. While we know the cause of this is desire and attachment, we don't know how to get rid of them. It has become our habit, our vasana. Hence sadhana or practice is needed. Repeated and regular practice of yoga on a mat at a fixed time of day/week is a small step towards a lofty goal of reaching lasting happiness.
As Swami Vivekananda says, we need a positive habit to get rid of our negative habits. Hence regular practice of yoga.