Sacred clay pouches (class 4) are made by using class 4 sacred clay on a Creation kiln. They can be made with level 60 summoning and require level 60 summoning to use. They are level 93 and act as a Beast of Burden with 18 inventory slots. When you use their Special Ability (you must have at least 1 Scroll in your inventory for this) they will deposit all of the items currently in their inventory to your team's base (Blue base if you are on the Blue team, Red base if you are on the Red team). For the most part familiars are used to quickly deposit all of a players items into his or her team's base just before the end of the game for the extra points. However, some players do use the familiars to aid in fighting though typically only class 5 familiars are used for this because the lower classes are usually unable to inflict any significant damage before the end of a fight. This of course would depend on the levels of your opponent an is not to be ruled out if many of the opposing players are of lower levels.
When you die any familiar you had summoned will disappear and will drop any items they were holding on the ground where they were standing when you died. Many players, particularly mages, though theoretically anyone could use this tactic, use familiars to their advantage in this situation. When the familiar disappears and drops the items it is holding the items, for a short time the items are only visible to you (similar to if you were to drop the items yourself). For this reason many mages use familiars to hold extra runes so that if they die they can run back, pick up the runes, and resume fighting without having to make more.