Anxiety can be a good thing yes, but! it also does more bad than good.
But agree with shahin that anxiety makes you control situations, and know how to deal with things over and over again day in and day out..
but some situations can make a person too anxious or (too BAD anxious ), well in that case a person should avoid being in such situations, because in nature that limit is what is described as a trigger to a life threatening situation like when a person sees a tiger or lion.. same thing it gives you the power and ability to over come the situation.. like run fast, or hit strong, so on.. but if you are in a civilized place the only thing you can do is take deep breaths through your nose and out of your mouth ,
and if that is a stressful thing in public i advise leaving a small gap in your mouth open where you can exhale from.