It's like any insurance gives peace of mind, on my last E61 I bought warranty from Car Care Plan through my motoring association cost around £350, later my computer started throwing up funny messages as they can do when the battery voltage is low, I had my local friendly BMW dealer check it out and turned out that the alternator was over-charging and shutting down, cost to repair including diagnosis £1200+.
I thought thank G..! I had bought warranty, that's when I found out why it was so cheap Car Plan said we supply our own alternators but if you insist on a genuine BMW we will only contribute what our own(not genuine) would cost us, having been in the motor trade all my life and knowing what problems can arise from non genuine re-con parts on BMWs' I opted for genuine plus I also got a courtesy car from the dealer
To cut a long story short Care Plan sent an assessor to the BMW dealer to check out the claim, the outcome being that Car Plan paid the total cost, I don't why, but I do know I shall be buying BMW Warranty next time.