Why 'file' local variable is obsolete?

[Przemysław Wojnowski]

Hello everybody,

Having the following code with 'file' as local var:
(defun file-processor (filename)
  (let ((file (expand-file-name filename)))
    (message "Processing a file...")))

Compiling the file gives warning:
In file-processor:
aoeu.el:2:10:Warning: `file' is an obsolete variable (as of 25.1); use 'file
    instead

I do not read every single email on the list, so maybe I missed something, but 
why there are byte compilation warnings for some variable names?
For example it is for 'file', 'tree', 'status', which are quite common local 
names. Moreover the message is not too helpful.

Thanks,
Przemysław