Navigation & Mapping
The most basic answer to the question what is GPS used for in the military is for straightforward navigation. From far-flying jetplanes to ships across the sea to jeeps in the desert, military personal have to know exactly where they are at all times to get their tactics to work right. It's easy to get lost in the vastness of the world, which is something that no military can afford.
How the military is equipped with GPS may vary from handheld portable devices for ground soldiers to fully loaded satellite dishes on mobile units. Either way, it's a powerful technique to really make tactics work and give the GPS-enabled troops the advantage over those still relying on maps and compasses.
Furthermore, having it all digitalized and online means that updated maps can be quickly supplied to all relevant parties. This is especially true on unfamiliar terrain, making it so that native soldiers lose something of their edge.
Routes through foreign cities can be quickly charted, nearby resources plotted out, bases mapped, troop movements displayed, rendezvous points marked within a few feet of accuracy meaning every piece of the military machine is quickly accessible for any soldier who needs to know to make the most out of what they're doing. After all, logistics are the backbone of any military, and GPS technology forms a powerful part of that.