People smurf for 1 of 2 reasons:
Reason 1: They just want to play with their new-to-league low level friends. The decent people in this group intentionally don't play at their best (unless it's obvious they're against another smurf) and smurf solely to not force their friends into having a horrid experience playing against people with 1000x more experience than them.
Reason 2: They're fuckin' bad, and are stuck at some rank on their main, and go to a low level account to ruin the fun of lower level players and pretend they're not complete garbage themselves. These are the people that QQ at you for being bad at level 4. Meanwhile, every time they play on their main, they're completely ass-punked because they don't realize they suck.