The unity manual actually answers your question:
Shadowmask Lighting Mode provides the highest fidelity shadows among all the Lighting Modes, but has the highest performance cost and memory requirements. It is suitable for rendering realistic scenes where distant GameObjects
are visible, such as open worlds, on high-end or mid-range hardware.
The shadowmask lighting mode allows Unity to combine baked and real-time shadows at runtime, and to render shadows in the far distance. It does this by using an additional lightmap texture known as a shadow mask, and by storing additional information in Light Probes.