If you don't do that, you usually can't even get those jobs. It's a stupid system. It's basically what became of the medieval guilds, in modern times.
People like to connect our low youth unemployment rate with this system, but I believe in reality most of these people are attending some form of education or job training program, so they don't "count" as unemployed. When they're done with all that, they're so old they don't count as "youth" anymore. That trick only works when a country can afford it, though.