On commodity PC platforms there are both upsides and downsides of having runtime firmware in a special mode like SMM (or the secure world on ARM64 which is a whole other topic). For hardware errors (e.g. mapping row/bank/column to PA), platform workarounds, certain platform power states, power fail notifications, backwards compat, (unfortunately) extending AML by writing to port B2, etc it can be beneficial for hardware+firmware vendors to have a runtime execution environment that doesn't require shipping OS drivers. Lately the downsides of this are growing and historically security hasn't been great.

That being said, there are benefits of having SMM protect firmware integrity from a malicious/compromised kernel. On most AMD64 platforms, SPI flash can only be written to by SMM and that is a foundational piece of the PC security model (FW update, authenticated UEFI variables, etc). This has nothing to do with copy protection, DRM, or backdoors.