Using Smite on the Greater Murk Wolf will summon a nature spirit that grants sight over the nearby crossroads for 75 seconds. Whenever an enemy champion comes in range of the nature spirit, it will chase them for a short duration. This bonus can only be claimed once per spawn.

Additionally:

It actually also pings and throws up a flag on the mini-map when it has spotted an enemy, though I've observed this can get lost in the bustle of a 5 on 5 Summoner's Rift game.

It will not detect stealthed enemies.

Usage:

Vision is useful so this ward is useful, however you should be placing actual wards at your jungle entrances, which can make the roaming ward superfluous.

It is more useful for obtaining vision in your enemy's jungle. E.g. try to smite the opposing team's wolves. Note that if you need to get in and get out quickly, you can simply smite the Greater Murkwolf in passing instead of trying clear the entire camp.

For your part, it is not a bad smite if you are going back to base and are lacking in vision on the blue side of your jungle. Otherwise, simply keep the camp clear so that the enemy jungler cannot use it to gain vision in your jungle.

Another use for it is to watch the path that it travels while following an enemy in your jungle. This can give you clues regarding what lanes are about to get ganked or where the trespasser is planting wards.