What's the point of `save-excursion` twice in `copy-to-buffer`?

Question

Here is the official definition of copy-to-buffer in Emacs 26.1 (the doc string part is omitted for brevity)

(defun copy-to-buffer (buffer start end)
  (interactive "BCopy to buffer: \nr")
  (let ((oldbuf (current-buffer)))
    (with-current-buffer (get-buffer-create buffer)
      (barf-if-buffer-read-only)
      (erase-buffer)
      (save-excursion
        (insert-buffer-substring oldbuf start end)))))

I don't understand why it's necessary to use save-excursion in conjoint with with-current-buffer? As far as I'm concerned, the former is equivalent to a combination of save-excursion and set-buffer, so the current point would be saved and restored anyway.

In other words, I'm wondering why don't the Emacs implementers use the following piece of code, which after some experiments appears to be functionally identical to the current one.

(defun my-copy-to-buffer (buffer start end)
  (interactive "BCopy to buffer: \nr")
  (let ((oldbuf (current-buffer)))
    (with-current-buffer (get-buffer-create buffer)
      (barf-if-buffer-read-only)
      (erase-buffer)
      (insert-buffer-substring oldbuf start end))))

Answer 1

Why do you think so? with-current-buffer does not do a save-excursion. Its purpose is just to temporarily change the current buffer (i.e., set it to some buffer and at the end restore what was the current buffer beforehand as the current buffer once again).

This is all that macro with-current-buffer does:

(defmacro with-current-buffer (buffer-or-name &rest body)
  "Execute the forms in BODY with BUFFER-OR-NAME temporarily current.
BUFFER-OR-NAME must be a buffer or the name of an existing buffer.
The value returned is the value of the last form in BODY.  See
also `with-temp-buffer'."
  (declare (indent 1) (debug t))
  `(save-current-buffer
     (set-buffer ,buffer-or-name)
     ,@body))

The purpose of save-excursion is instead to save and restore point and which buffer is current.