When adding more users on one side of a platform (like drivers) makes the product more valuable for the other side (like riders), and vice versa.
Uber's growth in any new city followed the same pattern: subsidize drivers until wait times drop below five minutes, which pulls in riders, whose demand pulls in more drivers, which drops wait times further. Each side was useless without the other, and the company spent over $30 billion in cumulative losses through 2020 to buy that liquidity in market after market.