The secure element is the core piece that protects against brute force attacks and physical attacks. It is what enforces brute force throttling and also makes tampering far more difficult. We also use it for other features like in Auditor.

It is a hardware requirement of GrapheneOS to have one and for the device to support an alternative OS using it. That is one of several reasons we use Pixels and this note is an example of the benefit. 

 If someone wanted to brute force a device that is BFU, they need to exploit that secure element in addition, rather than just the phone. Very old Pixels we do not support anymore have secure element exploits as per the Cellebrite documentation.