Here's why:
* Unown's rarity: While they are not as rare as some legendary Pokemon, encountering them in Soul Silver requires a bit of luck and exploration. They are scattered throughout the game in specific locations.
* No in-game purpose: Unown are mostly collectible and have little practical use in battles due to their limited movepool and low stats.
However, catching all Unown can still be satisfying for completionists:
* A sense of achievement: It's a challenging task that requires dedicated hunting and exploring.
* Completing your Pokedex: Completing your Pokedex is a common goal for many Pokemon players, and Unown are a significant part of it.
So, while there is no specific reward for catching all Unown in Soul Silver, the accomplishment itself can be a satisfying one for players who enjoy collecting and completing their Pokedex.