Q:

What is the purpose of a Smurf account in League of Legends? 

Possible Duplicate:
What is Smurfing in online games?  

I understand that a Smurf account is a less-used account owned by a player with a much better account, but I don't understand why you'd want to have one in League of Legends... It's not like other games where you can use a Smurf as a Mule to hold your less-important items.  What is the point of an accomplished 1400~ elo player having a Smurf account? 

A:

High-ranked players (generally around level 30) will use smurf accounts for a few different reasons, some reasonable, others less-so:

Playing with less-skilled friends, or with friends who are new to the game.  The high elo player doesn't want to instantly drag that person up against the type of opponents that they typically face, so they create a new account and play at a lower level. 
Spending time with a new champion (or champions).  This is less-true of very high elo players, but some players do want to spend time learning how a different kind of champion works.  They will create a smurf account and exclusively play a single champion in a low-pressure environment.  This is far less popular now, of course, with the Coop vs. AI games being available. 
Pub-stomping is also, unfortunately, a reason someone might want a smurf.  Some people like the utter domination that comes with being so much more skilled at the game.  This practice is often frowned upon, but does still happen. 
Pro players sometimes create smurfs to mask their identity.  If you watch their streams, you'll notice they're constantly receiving friend requests, etc.  And sometimes they just don't want to deal with it.  This reprieve is usually short-lived. 
As user22193 pointed out in another answer, another reason to create a smurf is if you change server realms.  For instance, Team Solomid was high-ranked on the NA servers, but created EU-W accounts and had to get their accounts up to level 30 and their Elos back to a stable position near the top of the ladder. 
M2tM: Smurf accounts can be played on if you decide to dodge a ranked game before it starts.   If people in a ranked game lobby are making you nervous about your win chance and you close the game you are unable to queue for another game for at least 30 minutes.   Your smurf will be available to play on, however. * This is now 5 minutes for the first dodge, then it ramps up. 

There are other potential reasons (that should just be edited into this answer), but those are the primary causes for smurfing in League of Legends.