It's the speculation, unregulated algoritmic trading [with nobody knowing exactly what is happening and who carries the responsibility], speculative trading on things that aren't actually there/not backed by enough assets and lack of regulation that is part of blame for the 2009 banking crash.
We pay for that, still.
The great depression was also caused by a stock market crash. It's wealth that's often not backed by assets, or at least not in a 1-1 manner. So when the debts are called in, or there's a tumble-effect it can all fall down badly.
And what exactly is the link between stock brokers and brexit? [alu foil hat alert, but it's somewhat relevant if proven correct]
I am not anti-stockmarket, but hedge funds with all sort of complex constructions, market panics etc. do make me somewhat nervous.
Of course the stock market is also related to euro clearing and some functions are perfectly healthy, it's not black and white.