WhatsApp is a closed silo — you can do with it only what the company chooses to let you do. Like Facebook. They can start placing annoying adverts, restrict what external systems you can integrate with, introduce charges for services that were once free, and change their terms of use in any way they like at any time.
Yes, they provide a useful service, for free. But you get sucked in, then you get locked in.
Matrix is Open (as in freedom, liberty) — you can move to another Matrix service provider and still talk with everyone, no matter which Matrix service provider they signed up with. Your work, university, or friends can host their own Matrix server, control their own data, modify it to talk to other systems, and customize it as they wish.
If you care about freedom of communication, use Matrix!