what's the reasons for having eunuchs in the Roman Catholic Church?

The practice of having eunuchs in the choir was to preserve the voices of the boy soprano singers. Castrating the boys before they went through puberty kept their voices from changing. The problem was that not all of the boys kept their voices. The problem is that castration does not keep the body from growing and developing, although it doesn't develop normally. Either some boys were too close to puberty when they were castrated and they lost their voices, or some of them lost their voices anyways as they continued to grow. Only a few of the castrati singers actually kept their pure sopranos. Read Anne Rice's Cry to Heaven. She actually did some pretty thorough research on the topic.

Eunuchs were made so for art, not sex or religion. Their testicles were removed before maturity to prevent the testerone growth that would change a boys voice and turn him from the angelic appearance of a boy to the rougher textures of a man.

You are speaking of contaltos - boys were castrated before reaching puberty so their voices would not change. This is something that the PARENTS of the boys did so the beautiful singing voices of the boys would not change. This practice was never endorsed by the Catholic Church and as you noted was banned by the Church

It was based on the Scripture passage...

Mat 19:12 "For there are eunuchs who were born that way from their mother's womb; and there are eunuchs who were made eunuchs by men; and there are also eunuchs who made themselves eunuchs for the sake of the kingdom of heaven. He who is able to accept this, let him accept it."

This is also one of the reasons that Priests remain celebate.

1Co 7:32 But I want you to be free from concern. One who is unmarried is concerned about the things of the Lord, how he may please the Lord;

1Co 7:33 but one who is married is concerned about the things of the world, how he may please his wife,

1Co 7:34 and his interests are divided. The woman who is unmarried, and the virgin, is concerned about the things of the Lord, that she may be holy both in body and spirit; but one who is married is concerned about the things of the world, how she may please her husband.

1Co 7:35 This I say for your own benefit; not to put a restraint upon you, but to promote what is appropriate and to secure undistracted devotion to the Lord.

because most people are lovers or fighters and if they can't have love they might decide to become the best fighter they can be. ie they will be more aggressive. cause ancient people hated ROMANS

this was a way to enable choirboys to sing in higher tones.