The bell tower serves as a landmark in Ecruteak City, and it's closely associated with the legendary Pokémon, Ho-Oh. You can find Ho-Oh by completing the "Bell Tower" event, which involves getting the "Golden Ticket" from Kurt in Azalea Town.
The event is triggered by talking to the old man near the bell tower, after you have caught all 100 Pokémon in the game. Ho-Oh will then appear, and you can attempt to catch it.
However, reaching the top of the bell tower itself is not part of the game's design.