Except for the refferal system, when you lvl your smurf till 5th level, i don't understand this. Is this so you can have a nice wins/loses ratio? Is this because you got too deep in elo hell and it looks like there's no way getting out so it's just simplier to create a new acc? Is this because you can roflstomp noobs? <- I' ve tried this once, just to see the difference between new players and me- can i faceroll when playing against them But i noticed, that few of those low lvl players really play like low lvl players ( I rember one gp in my first match with smurf account, who from lvl 1 went to baron and said he will guard it(baron) from other team, probably he was a troll lol).
tl;dr
Why there's a need for the smurf? (except to get RP from referal system).

1. because their main got banned
2. to make their ego go bigger, when they can kill all noobs at low levels and carry games by themselves.

Or to enjoy non-full meta games, where people actually dont rage that hard at each other.

Because I play with my friends on another server ^^

Top players use smurfs when they don't want all the in game attention that comes from being a celebrity. Others use smurfs because they think they're pros when they really aren't. Others think they can escape "Elo Hell" by playing on a smurf and having a better default Elo, only to drop back into "Elo Hell" due to their bad attitude, which is what got them there in the first place.

Because at top elo que takes 30 minutes?

Smurf #1: If the person is a bad - mediocre player then it'll show; they'll blame their team's loss on everyone else but themselves and will always run in 1v1 or 1v2 without assessing the circumstances first, and complain if they get ganked or lose the fight, saying such things as "LOL YOUR CHAMPION IS SO EASY TO PLAY", "LOL YOUR CHAMPION IS SO BALANCED" or "LOL YOU NEED 2-5 PEOPLE TO KILL ME".

These smurfers will generally have a bad attitude and cannot handle the fact they have more losses than wins on their main; because of their bad attitude there's a large chance they may have been banned before too.

Smurf #1.5: People who have been banned before.

Smurf #2: People who want to duo queue with a friend new to the game and don't want to queue a lvl 30 account with a lvl 1 - 10 who doesn't know how to play and has only been schooled on bot games so far. People sometimes solo queue with these accounts because they feel compelled to goof off at low levels and do the same thing as Smurf #3.

Smurf #3: People who are scared of using free week / new champs and want to try them out without being yelled at for playing them badly.

Smurf #4: People who just want to use troll builds for a laugh, but not necessarily throw the game. For instance I created an account so I could play as AD Ziggs. Won too much though so I got smurf detected and people started yelling at me for going AD Ziggs.

Smurf #5: People who want gimmicky names but don't want to spend the money and / or change their main account's name for whatever reason. I have a plethora of accounts named after the Metal Gear franchise on the US server. I also own Rumble Bot. However this is the only account I own on this server.

Smurf #6: These aren't actually smurfs, but because some people switch in between servers because different friends play on different servers; they'll need to create another account on another server and will obviously not be new to the game.

Smurf #7: The rare individual who's genuinely bored of high elo games where the meta is usually a passive 20 - 30 minute laning phase last hit whackamole game. Not saying all high elo games are like that, but a lot of them are. However this person can have habits similar to Smurf #1, and tend to come in packs and play in premades with others like themselves. Not to mention the queue times are ridiculously long.