It's fame comes from the simplicity of its construction rather than its utility elsewhere in mathematics. For example, Graham's number is pretty famous but it'

It's fame comes from the simplicity of its construction rather than its utility elsewhere in mathematics. For example, Graham's number is pretty famous but it's more of a historical artifact rather than a foundational building block. Other examples of non-foundational fame would be the famous integers 42, 69, and 420.