The general idea behind secure boot is to prevent malicious code from being ran on your PC.
The idea behind requiring it for BF6 is to prevent cheating.
That said, I think it's a ridiculous requirement for a game. There is other anti-cheat methods that do a fine job. Does the secure boot requirement do a better job? I'd assume so, but it also means there's just regular people that literally can't play the game. And if your anti-cheat blocks regular people from playing the game, your anti-cheat is not working correctly.
For tech literate people, it's not a big deal to check if your partitions are GPT or MBR, and if they are MBR to convert them. Then to go into bios to turn on TPM and secure boot. This however would be difficult for a lot of people that find the command line scary (it's fair to be scared by it, you can mess up your computer).
Does Win 11 require GPT? Yes.
Is Win 10 being end of lifed in a couple months? Absolutely.
But is it a games place to force you to make changes to your bios and disk drives to play? In 1995 or 2005, sure it wouldn't be too unreasonable, but in 2025 I think it's an expectation that is unrealistic and frankly ridiculous. Some people don't even have a TPM but their computers are fine, why should they have to purchase a new mobo or an external TPM just to play a game when their computer and other games of similar spec perform fine.
While I do have a TPM and can turn on secure boot, I don't appreciate the expectation and the likely trend that will follow if battlefield doesn't remove this requirement. They can remove the requirement and still have a strong anti-cheat. That said, EA is not known for listening to feedback, so I am sorry to all of the people that want to play the game but can't due to arbitrary hardware requirements.
