Mindfulness is said to be useful everywhere. That means that we can always do it. We can practice it in every circumstance.
Too often, people think that they need to hold a specific focus, such as the breath, in order to practice mindfulness. But we can experience everything with mindfulness. Are you experiencing pain? You may not be able to focus on your breath, but you can experience pain with mindfulness. Are you experiencing joy? You may not be able to focus on your breath, but you can experience joy with mindfulness. This experience, not a different one, is where we most easily establish mindfulness. Mindfulness is relevant always. It is how we make this very moment into the next step on the path to the end of suffering.