It's pot luck, but the idea of having a cross section of the magistrates that roughly matches the local population is that there will at least be a reasonable chance that out of the 3 you normally get in a sitting one of them will be nearer your background.
Magistrates also talk to each other and learn from each other, so having members of the bench from different backgrounds means that they do pick things up.
This is nothing to do with BLM or any other rubbish, it's about trying to make sure that the judiciary is able to understand the people it's dealing with as well as the law.
For example is someone turning up to the court in grubby clothes being disrespectful, or simply coming to the court straight from work?*
Or is someone turning up in a "funny" looking outfit not bothering, or wearing something that they consider respectful.
Having members who are mainly older and from a specific group means they don't have the same life experiences or may not be able to understand some subtleties of parts of the society they are dealing with, diversity therefore is not only good but necessary for the group as a whole to do the best it can.
It's a bit like the police force, where the move from mainly one group making up most of the officers to a wider variety led to things like crimes against women being taken more seriously, and women (and ethnic minorities) being more willing to talk to them about sensitive stuff.
*One serving magistrate posted a great example on his blog of how someone who at first glance may be being disrespectful to the court, had in fact already appreciated the sentence he was likely to get (driving ban) and had managed to change his job to allow for that, by moving from a driving role to a yard job at his work and was already working longer hours to make up for the loss of income (this is from memory). So far from being disrespectful of the court, he was showing he was serious about understanding what was going on.
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