In the most basic sense, anxiety (like joy and other emotions) is a normal and necessary aspect of life, and our desire to avoid or escape from situations that we associate with either danger or aversive experiences (e.g., anxiety, disgust, frustration, loss, etc.) is usually helpful or adaptive.
From a biological perspective, the experience and expression of emotions evolved before language and helped mammals, and humans in particular, to communicate a sense of safety or danger to each other. By doing this, emotional beings are able to learn from the experiences of others, instead of always having to make direct contact with the potentially catastrophic consequences of their own actions. For example, a young toddler who has not yet learned to speak nevertheless appreciates the urgency in his mother’s facial expression and tone of voice as he gets too close to a hot stove, and quickly hurries away from the source of danger, as a result.
When a person is faced with real or perceived danger, the body reacts by preparing to either confront or flee from the threat. This is commonly referred to as the “fight or flight” response, or the ability of our nervous systems to rapidly mobilize bodily resources in order to survive an imminent risk. Many of the most common physical symptoms associated with anxiety (e.g., accelerated breathing and heart rate, heightened alertness or vigilance, sweating, etc.) are actually normal physiological processes essential to the fight-flight response. Given this, the function of anxiety as a kind of “alarm system” is vital to the overall survival of our species, even as the experience is felt as unpleasant or intolerable by the individual.
Although a degree of anxiety is necessary for normal functioning and the assessment of the danger, terms such as "panic," "phobias," "sensory aversions," and “anxiety disorders” refer to instances in which a person's situational appraisal and/or bodily reactions are inaccurate. In other words, the tendency of the "alarm system" to "ring false alarms" is getting in the way of life. Typically, treatment is indicated when a person is experiencing a (1) significant or debilitating level of fear or disgust, (2) a tendency to think catastrophically and significantly overestimate danger, and (3) excessively avoid or look to escape from particular experiences, situations or people. From this perspective, an anxiety disorder is a lot like having a malfunctioning alarm system, which requires retraining rather than "fixing."