Answer: Halo effect
Step-by-step explanation:
In psychology the halo effect is a sort of bias in which a person tries to impress others through his/her overall appearance. The more good your appearance looks, the better your chances would be in impressing others.
In the question, many people wear formal attire because that signifies a level of good appearance that would land them a job. Even though it is not require, the candidate are doing that to get the attention of the interviewer in a way that would help them get the job.