There is one other reason for a smurf-like account being created. With all the different servers League of Legends is being played, if you want to play on a different server, then you ultimately have to create a new account. This happened a few months ago when North American team, Team SoloMid, moved to the gaming house and decided to all create EU West accounts. The end result is you had 5 of the best players in the world creating lvl 1 summoners in EU West and generally destroying the competition in Normal games until they finally reached Summoner level 30.
On the Less-skilled friends, it is simply the game is more fun when you win. So if you want your friend to play with you, you want to do what you can to let your friend win that first game, and one way to do so is to lower the competition of who you face by being on a lower level account.
On the second point, I am guilty of this. My first summoners (main) I played most tank and tanky support. Since then, the metagame changed, and I had no experience playing carries. In fact, I lacked the runes and champions that qualify as carries, so I decided to create a new summoner (smurf) to only buy things for carries and focus purely on that style of play.