Well in an evolutionary sense anxiety is the anticipation of danger, it's an ability unique to that humans that in nature would have allowed us to plan ahead and avoid harm in the future. All people will still experience anxiety to some degree and that can be a good thing, to a certain point it still improves work and academic performance.
It can become a problem though when a person worries about a perceived threat or just worries about worrying, to the point where they have difficulties maintaining a productive lifestyle. This is when you might say someone has an anxiety disorder, theories that explain anxiety disorders might explain it as a learned response or biological. For example someone might develop social anxiety because someone laughed at them in a social situation when they were young, this can generalise to other situations. But there doesn't always have to be such an event, some people may just fear an imagined event or be genetically predisposed to anxiety.
This is why there are multiple ways of treating it, if someone has learned to anticipate a threat in a certain situation then therapy may help them identify real and perceived threats or outcomes. Medication can also be used to combat over productive stress hormones. It might help to think that it's an important evolutionary trait that all people have and has allowed humans to thrive, though with the modern world which has become very complicated there are many things that can be perceived as threatening, which is why some people need help managing anxiety.
