Manual saves in games
tl;dw:
-Manual saving allows reloading to before any event in case you accidentally skipped something that cannot be seen again that playthrough (like missed dialog etc.)
-Manual saves allow forcing a save before quitting, thereby being certain that you're up to date without running around trying to find some way to trigger the autosave so it stops saying "5 minutes since your last save - really quit?!"
-An arbitrary (or at least reasonably high) number of manual save slots allow speedrunners to practice sections of the game at will, or just anyone who has any reason to want to rerun a section (see link below)
-At least one game accessibility site lists "a manual save feature" as one of its suggested guidelines: http://gameaccessibilityguidelines.co...
-Manual saving is a perfect example of providing for the player to "play the game the way they want to"; if you're going to let them be invincible then there's no reason you should try to prevent them having as many saves as they like.
-See the comments for people giving even more of their own reasons!