In our daily activities we often just follow our thoughts and emotions without questioning them. We might feel such a stress that we do not find the time to step back and check if it is really necessary to stress our self out etc.
This doesn't only apply to stress, it is the same for any type of emotion or thoughts.
So if you do not learn how to observe your thoughts and emotions, they will carry you away. You might find yourself full of anger, without really knowing why, or full of attachment to foods etc.
When practising mindfulness through meditation, we learn how to take a step back, how we are actually not our thoughts and emotions, we learn to distance ourselves, and we slowly gain control over them.
Now instead of easily following the emotion of anger, we just observe the anger, instead of blindly following a craving, we just observe our craving etc.
In the long run, this gives us more freedom, freedom to choose our reactions, freedom to act in a more positive manner, freedom to be someone we want to be.