The "Spacer plate" is also called a block plate. This is required to make the assembly SFI approved - theory being that a clutch explosion might be entirely contained within the bellhousing. Without the block plate, you could find shrapnel coming forward through the flywheel and even through the engine block. It's all worst-case scenario stuff, but if you've seen what a disintegrating clutch assembly can do at 7000+rpm, you very quickly see the value in a blow-proof bellhousing.
