Apart from the fact that they are the greatest creation known to humankind.
1. Minimum muscle use = hours of toasty warmth. "I thought both sides of an equation were meant to add up?!?!" I hear you say. Exactly. Intangible benefit my friend.
2. Provide, not only less, but ZERO bum-in-face action, unlike your your average pet cat, who, although warm, will take every opportunity to jump on your lap, turn round and stretch.
3. Whatever you leave it lying on becomes warm. Example you ask? Imagine lying all evening against a soft cushy pillow. Yes one side does become warmer, hence "flip to the cold side of the pillow", but imagine it's about minus 10 outside and you're lying against a hot water bottle which is on your pillow, and you decide you'd like the hot water bottle to heat up your front, OH MY GOD THE PILLOW IS SO WARM! NOW I'M WARM ON BOTH SIDES! See? Better.
4. Unlike Shania Twain's long list of disappointing men, hot water bottles do "keep me warm on the long, cold, lonely nights". Regrettably the don't cuddle you back, unlike that weird half a person-cushion that the Japanese invented, but the immeasurable happiness you obtain from a warm night's sleep nullifies this.That does impress me much. Uhn uhn, aw yeah yeah.
5. Pain relief. Deadly serious benefit now, the soothing effect produced is unbelievable, why waste £16.50 on a heat rub that makes you smell like smoky bacon when you can grab a hold of a hot water bottle?
6. What do dragons, radiators, fire, and acid have in common? All will burn you if you put your face on them. Hot water bottles will warm you up but NEVER BURN YOU. They know where the line lies and never cross it.
This post may or may not be inspired by the fact that the UK looked like this on Thursday: