Vim :help window
explains the confusion "tabs vs buffers" pretty well.
A buffer is the in-memory text of a file.
A window is a viewport on a buffer.
A tab page is a collection of windows.
Opening multiple files is achieved in vim with buffers. In other editors (e.g. notepad++) this is done with tabs, so the name tab in vim maybe misleading.
Windows are for the purpose of splitting the workspace and displaying multiple files (buffers) together on one screen. In other editors this could be achieved by opening multiple GUI windows and rearranging them on the desktop.
Finally in this analogy vim's tab pages would correspond to multiple desktops, that is different rearrangements of windows.
As vim help: tab-page
explains a tab page can be used, when one wants to temporarily edit a file, but does not want to change anything in the current layout of windows and buffers. In such a case another tab page can be used just for the purpose of editing that particular file.
Of course you have to remember that displaying the same file in many tab pages or windows would result in displaying the same working copy (buffer).