Using a single nominee lets billions of shares change hands by book entry without ever re-registering anything with a transfer-agent. The company's register shows one enormous holder; the real ownership chain runs from that nominee down through dtcc participants to brokers to customers.
The chain matters for voting and for corporate action elections, because each layer has to pass instructions along, and it is why deadlines a broker gives you are earlier than the company's own deadline.
Example: a company reports 220M shares outstanding, of which the register shows 198M held by the nominee. The other 22M are direct-registration and employee plan accounts registered directly.
Related: dtcc, street-name, direct-registration, transfer-agent, voting-rights