Without liquidation preferences BrewDog wouldn't have been able to get the investment. They may have been able to get a loan instead, but the interest would've driven them under that much faster and then the creditor, just like the preferred investor, would have priority over other shareholders.

Which isn't to say preference stacks (like debt stacks) can't get absurd when a failing company is doing anything it can to stay alive, but standard investor terms (1x liquidation preference) simply mean you're first in line to get your money back if the company is liquidated for less than the price you paid.

The self-dealing bit is generally already illegal and orthogonal to liquidation preferences.