What Use Is An F-call?

What use is an F-call? I'm about to go on a couple of trips and thought that it might be a good idea to have a radio with me, if nothing else, a great opportunity to talk to some locals on their local repeater. I have a portable kit, that is a Yaesu 857d with 2 x 26Ah batteries which goes forever using 10 Watts. My back of napkin calculation says that on 2m I can transmit continuously for over 12 hours with that - more realistically, if I transmit 10 minutes out of every hour, over 40 hours, though I've never tested it and likely the numbers will be different for HF using SSB with my Antenna Tuner plugged in. This amazing battery life comes at a cost, namely size and weight. The batteries fit in a medium size tool-box, and each battery uses more than 8kg from my luggage allowance, so, if I was keen, I could stick my radio and 16 kg of battery in my suitcase and take my toothbrush but I'm pretty sure my hosts would prefer I packed some clean clothes as well. I've been shopping around for a hand-held and I