The good thing about blockers in people starting puberty is that the effects are reversible. So if the client decides later that they wish to continue in there assigned gender they are withdrawn and with a little hormonal help normal puberty will take place.
About 25% of female to male young people decide to live their lives as female, and about 10% of male to female young people decide that they wish to continue life in their assigned birth gender. Puberty blockers allow the medical team to cater for these clients.