Because Flat is a global object, thus storing unnecessary results from previous function calls.

I don't follow, how does the end parentheses make it private?

I too not recommended using eval.. I think this kata trying learn us work on state machine.

Using eval would not be recommended if you can't control the input (For example 'user input' -> could be executed as code)
This is a Kata though, not a function that would run after some user input on an actual application.