From the RuneScape Wiki, the wiki for all things RuneScape
The Elemental talisman is an item that allows players to enter all four of the elemental altars which they must enter if they wish to craft elemental runes. Unlike all other talismans, it cannot be bound into a tiara or a runecrafting staff. The four elemental altars are Air, Water, Fire, and Earth. It cannot craft Elemental runes.
Elemental talismans can be obtained as drops from the monsters of The Abyss.
This talisman is not used often because people almost always craft each rune separately. In addition, as this talisman cannot be used to create combination runes it brings the usage down even more. An Elemental talisman can be seen on the shelves in the magic shop at Port Sarim.
This talisman seems to be for those who wish to save bank space. Instead of having four talismans in the player's bank, one for each element, they can have just one. However, it does slow down Runecrafting slightly, because the player cannot fashion it into a tiara. At level 50 Runecrafting an Omni-talisman and a Omni-talisman staff would save more bank space.