The sperm and the male hormone testosterone are produced in the testicles. If you remove them, the hormone production will be interrupted and with that their sexdrive will be strongly reduced. That will help to prevent following their urges and will help themselves to get free of their sick sexual compulsions
Sex offenders have something wrong with their mental state, not their lower regions. What needs to be helped is taking them off the streets and addressing their state of mind. Many of them have had difficult childhoods, or something tragic happen to them. It could also be they are underdeveloped in the brain and do not know better.
No, castration of sex offenders would not help save them from themselves. The desire for sexual assault against anyone is a lot more complex than the existence of reproductive organs, and no one would benefit from castration. Furthermore, it would begin to ostracize sex offenders in society further than they already are. Sex assault is a multi-layered issue, from societal to mental, and requires assistance on all fronts.