when I equip charms with such affix, it doesn't add up to my life pool, so what does it do then ?

yeah it does, in main slot. in acc slot, mods are nulled. it adds that much to your life pool

http://ladderslasher.d2jsp.org/index.php?g=1#Accessory%20Slot

The Accessory Charm is an additional Equipment slot reserved for a second charm. The Accessory slot however does not reflect stats that are on that charm, with exception to Enhanced Effect. Efficiency is reduced by 25% of the total displayed. This charm is used just like the main charm slot and is not reflected by your total inventory space restrictions like the rest of your equipment.
In the inventory, it is abbreviated as Acc. Charm