Economies of scale mainly.
And reducing the multiplicity of organisational interfaces in managing and developing a big station.
It also gives a long-term planning potential for investment, and taps a layer of big-project management and construction partners.
Can you imagine LM managing the Birmingham New St upgrade?
It also provides a consistent interface to government (eg city councils, PTEs, TfL etc).
NR management seems to be creeping down the size scale.
It's not entirely clear to me why large strategic multi-TOC stations like Bristol, Cardiff and Newcastle are not NR-managed if Reading is going to be.
Perversely, it also means TOCs often do not lead on the operation of their flagship stations - Virgin's biggest operated station is, I think Preston, not Euston, New St or Piccadilly for instance.