Liquidation multiples and preference gives the investor protection when things go bad (and also a kicker when it goes well). Let’s say you put in $100M and have a 2x liquidation multiple at a $4B valuation.
Even if the company falls apart and sells for $100M you still get your money back. If it declines to $200M you get all of that before anyone else.
In reality terms vary substantially. In this case, Doordash had just raised and didn’t need to raise again. Depending on how desperate investors were to get in, they might have gotten clean terms.