Confirmation tones are used to convey that the car registered a deliberate user action (e.g. a button press). For example whenever you press the unlock button on your key fob, it would quickly become annoying if the car announced "The car is now locked". In the case of confirmation, you know what the tone is signaling, because you just performed the action to create the tone (and don't need a lecture about it).

Mildly abrasive 'alert' tones are preferred for warnings over announcements because alerts typically require immediate action. You could follow the alert tone by an announcement but this is typically unnecessary because the the first time you hear it, the tone will be seared into your memory (e.g. you are messing with the radio and suddenly hear the lane departure warning or forward collision warning). And you really don't need to distinguish between them - they all basically mean "immediately allocate all cognitive resources towards driving this death machine safely".