There isn't a purpose.
It's an emotional response devoid of logic.
Why are you expecting a logical response from a group which has pushed it's cause forward so much on the basis of emotion.
We know there are strategists amongst prominent activists - who have actively admitted they wanted to force change under the radar (interesting to see Trevor Phillips comment on the active and deliberate dishonesty amongst some) but these are really the exception to the rule. The ideology is too incoherent and muddled to lend itself to the majority acting logically at this point.
The whole problem is most caught up in this area behaving in a cult like fashion and when cults are put under pressure they don't react well.
If we believe this movement is cultlike then we should take this seriously. They will actively believe they are under siege and that they really are being persecuted. This legitimises protests and actions to stray into criminality in various fashions. I would argue that this is actually a dangerous moment and one we shouldnt just dismiss.
I think there will be some who seek to make a name for themselves - we've seen this purity spiral on social media where people egg each other on and try and prove how committed they are to the cause. Already we are seeing photos in toilets and threats. The police need to be fully aware of what's happening otherwise they risk being embarrassed by an incident and us all asking why something wasn't done sooner because it was so obvious.
It's all very well starting threads saying 'look at this for balance', when we are seeing the manifesting of behaviour that we should be concerned about.
This post I'm sure will go down like a lead balloon but it's based on observation and legitimate concerns about demonstrated and probable behaviour. It doesn't matter if it's a minority of protesters causing a problem - that minority can still have a massive impact. We are in an era of lone wolf incel attacks as it is so this isn't an unreasonable position to be expressing concern about this.
Everyone has a right to protest. What they don't have is a right to threaten, harass or act criminally to persons or property. The difference matters. Behavioural patterns are well known and identifiable. I sincerely hope that the right conversations are being had where they matter right now, on the basis of that.