https://www.microsemi.com/product-directory/carrier-grade-nt...

  Grandmaster Clock [...] equipment designed for deployment
  in the mobile backhaul network, at macro base station
  sites, or small cell aggregation locations. [...]
  meet the stringent timing and synchronization
  requirements of 4G/LTE networks

In other words: Applications that need sub-millisecond-level precision, such as mobile phone base stations, where multiple towers coordinate their transmit/receive windows to avoid interfering with one another.