It's optional.
To get your player name to the scoreboards instead of a rolling Player####.
It's not necessary to play the game.
And the login is a user account for GDevelop as well as gd.games that is the same for all our games (or you can ofcourse make separate ones if you so wish), but basically it's for player account that has personal tags for high scores and in future for save games etc stored in cloud if I make our games to be saved.)