There is zero evidence that conversion therapy is needed or does any good. There is SIGNIFICANT evidence that it actively causes harm. There isn't even evidence that it does what it claims to do. It actively tells people that who they are is fundementally wrong. Done to adults this is damaging enough. Done to minor this is FAR more damaging and they have much less ability to avoide it. The practice itself actually uses shame to try and change people. This is psycological abuse.
Making the practice illegal actively protects minors. Anyone, particularly minors, should be able to go to a mental health professional and trust that they are being given eithical, helpful and sound care.