Minecraft wiki entry for Defeating_a_Nether_fortress, associated text: To get blaze rods, essential for brewing and crafting several interesting things, including eyes of ender and ender chests.
To get wither skeleton skulls to summon the wither or for decoration.
To set up a blaze farm or a wither skeleton farm to get the resources above.
To gather nether wart, also essential for brewing.
To get loot from chests generated inside.
To use as a naturally generated player base (not recommended, as hostile mobs constantly spawn here)
To challenge the player's combat skills.