Patents required because of ... patents
Presumably WD have patents required by Seagate and they have cross-licensed. Years ago, it might have kept others from starting a hard disk business. These days, a new hard disk business would cost billions to create, provide thin margins at best and never get investors to part with the start up cash.
What these patents do is protect against someone patenting the "idea" for a multi-stage actuator with no clue about how to build one. After WD/Seagate do all the development, buy the tooling, get a product to market start getting a return on investment the person who patented the idea could cash in.