At a lot of the USA schools (the big ones, that is) the bars are a mandatory part of the uniform.
The colour of shirt differentiates between student and instructor - but the main job of the bars (at the last professional place I worked at) was to advise the "dispatch staff" of the level of supposed competence of the person who was asking to be given the keys to an aircraft.
At a school with over 400 students - of which dispatch personally knew, maybe, twenty - it was a useful system to stop a pre-solo student being given the keys to a King Air.
The system only failed when a 'two bar' dispatch intern told (three bar) instructors that they did not have the authority to cancel their own training flight due to the weather (humongous thunderstorm outside at the time). According to dispatch, you had to have four bars to be able to cancel your own flight (of which you were the Captain). Go figure.