While some BIOS files are "Region Free," others are tied to specific territories to match the hardware they were extracted from: Dreamcast - RetroPie Docs
Here is the emulator trick: Because you are emulating, you can swap BIOS files depending on the game. Want to play Shenmue II (which never officially came to the US on Dreamcast) in English? You might find yourself booting into the European BIOS to run the PAL version, or using a patched US BIOS. sega dreamcast bios files
Think of the BIOS as the console’s operating system. When you turn on a Dreamcast, the CPU doesn't instantly know how to read a GD-ROM disc. It first loads the BIOS, which: While some BIOS files are "Region Free," others