As Max says its primarily a HR driven policy to recruit cadets.
a) A proportion of the cost is recouped through lower pay.
b) Give me a child until he is 7 and I will give you the man - the airline can control your training from day one.
c) You are less likely to leave the company as you can't afford to for the first 5 - 6 years and after that the loss of seniority is too much to bear.
d) Flexibility. They need some new FO's up in an unpopular base - fine, send the next batch of cadets, they ain't going to argue. Need 'em back in two years time, fine they still ain't going to argue. Try that with a direct entry pilot perhaps ten years older with a wife and two small children...
e) It shores up the empire of HR.
WWW