E.g. a company raising $5m Series D at a $30 million valuation, and which already has $10m in preferences. First you have to adjust the valuation because that $5m will be the first dollar out and might even be participating preferred - so they are getting 1/6th of the company for $5m but they are also getting basically a $5m note payable.
If you asked them what they would pay without the preference maybe it’s closer to $15m. Then you also have to adjust for the other outstanding preferences. So the common shares (which are likely to be even further diluted before they become marketable) in that case are presently nearly worthless.
Most people do not adequately consider the impact of preferred share preferences, future dilution, taxes, and marketability. These horseman turn a decision which might seem like at first glance to be, “Why give up the chance to participate in a future equity event?” into something more closely resembling a suckers bet.