Before the new command engine, it was possible to type /kill in the chat and it would run the command on yourself. Now specifying a selector is required, so you must type /kill @s or /kill @p (returning an "Unknown command" error otherwise). This isn't a huge issue, but being able to use the shortcut is more convenient.

Note that, for other commands where specifying the player is optional (which defaults to yourself when not specified), this behavior didn't change, e.g. typing /gamemode creative changes your own game mode without the need to include @p or @s.