I've been reading a bit about speaker design but I haven't been able to find what purpose the internal volume of a speaker cabinet serves. From what I've read, having an internal volume causes extra considerations like needing to deal with internal reflections and requiring bracing to reduce the cabinet resonance; though I am sure there are good reasons for it I haven't been able to find specific answers addressing this.
For example, why can't a speaker enclosure simply be a block of MDR with just enough space cleared to fit a driver and maybe a small vent to for air to be pushed/pulled? Is internal volume just like an expanded vent that improves low-frequency rendering by allowing a larger volume of air to move, and it's better/more efficient to make than say just a very long vent, which could resonate at much more detrimental frequencies (but couldn't those resonances be mitigated by bracing)?
I'm pretty curious about this so I'd love a good answer.