Well its a mixed blessing really, the biggest thing is to basically funnel new players into playing a less diverse champion pool at first (The weekly free 10s) this allows the player to refine their skills with a few select champions instead of having them run wild for 30 levels.
Also, with the 450 champs (the ones you should master first) each one teachs a core aspect of the game. Annie teaching last hitting, Poppy teaches positioning, Garen teaches target prioritization, and the things go one.
Lastly champions cost IP (or you could buy them with money) to prevent decision paralysis. Most people when joining a new game, that are given 140 options right out the bat will feel daunted and not know what to focus on or what to do.