Generation. Nether fortresses generate in all Nether biomes. To do so, the game splits the Nether into regions in which one of either a fortress or a bastion remnant can generate. The regions are 432432 blocks in Java Edition and 480480 blocks in Bedrock Edition.
Nether fortresses are one of the essential structures since they are the only places you can find blazes, which drop blaze rods, which are essential to beating the game. Each nether fortress spawns with two blaze spawners or blazes can spawn naturally around the fortress.