The herb sack can be manually filled by using the left-click 'Fill' option, which will deposit any grimy herbs from your inventory, unless it is full.
It can be opened or closed with the right-click options 'Open' or 'Close' respectively. An open sack will automatically deposit any grimy herbs that are picked up by the player, as well as those harvested from a herb patch.
The 'Empty' option deposits herbs into the inventory, starting from the lowest level herb, and with the bank open it will deposit all of the contents into the bank.
Using the 'Check' option will list all currently stored herbs within it in the chat box.
The herb sack is useful for killing monsters that often drop valuable herbs, such as chaos druids, flesh crawlers, aberrant spectres, and deviant spectres, since it only takes up one inventory slot. The herb sack can also be used when lighting incense burners during open altar house parties, reducing the frequency of banking or unnoting Marrentill at Phials.
Herbs stored in the sack are not lost upon death. Herbs will stay in the herb sack upon death, unless the player has untoggled this setting.