Some people have a deeper religious devotion than others. Those with a particularly strong aptitude for prayer and the consecrated life should have an outlet for their gifts. Monasteries provide that outlet.
A typical rejoinder for those who oppose monasticism is that it isolates the believer from the outside world, thereby missing opportunities for evangelism. I find that objection strange because (A) it's usually made by someone who has never attempted to evangelize others and (B) it presupposes that a charism for prayer and devotions is somehow the same as a gift of evangelism.