Why is `length` defined on numbers?

I discovered from a typo today that the `length` method is defined on numbers:

``` julia
julia> length(5)
1

julia> length(pi)
1
```

It's defined here: https://github.com/JuliaLang/julia/blob/master/base/number.jl#L12. Why is `length` defined on numbers? This seems counterintuitive. Numbers aren't iterable in any way after all.
