It's very strategically located, dominating the central stretch of the Kingsroad, the northern approaches to King's Landing, and much of the southern Riverlands, which explains why so many armies use it as a base of operations. Its also so huge that it can comfortable house any army that any Westerosi lord can muster.
In peacetime, the title comes with some of the wealthiest and most productive lands in Westeros, and is well-positioned for trade. Harrenhal is also the largest castle in Westeros and was once the "seat of kings", and is one of the only great castles in the realm that is not ruled by an ancient house that's had for millennia. Being granted Harrenhal can turn families like the Lothstons or Whents, landless household knights, into one of the Great Houses of the Seven Kingdoms overnight. There's no other title in the world that can lift a man that high up the ladder so fast.