A student may want to add a past course to their main course menu if they are taking a higher level version of that past course. (e.g., took Spanish I, now taking Spanish II and would like to review Spanish I material without scrolling through 30+ other past courses). Ease of access makes the users experience much more enjoyable.
This is what students see now on the desktop version of Canvas:
It should also be noted that this feature was implemented into the mobile version of Canvas months ago.