Defvar not called at top level

[Arthur Miller]

Just a question: why is it important to call defvar from top level?

I would like to have some code that generates some other code, amongs that to
declare few global symbols with auto generated docs as well.

If I call those from a defun/macro normally I get that warning that defvar is
not called at top level; so I guess it is for some reason a bad practice? So if
I generate some code that will be called at some later time that uses defvar I
guess it would be equally bad practice.

I just wonder, why is it demanded to call defvar as top level form?