I'm not really deep into the topic, so this may be wrong/inaccurate - but I've read that puberty blockers were actually introduced as a way to solve the problem of children having to make life-changing decisions.
A successful HRT becomes much harder after puberty.
On the other hand, performing HRT on children is plain unethical, for all the reasons discussed in the rest of this thread. (And btw something the trans community is advocating against - because forced sex reassignment had been standard practice for intersex children for decades. The trans community is demanding to end this practice.)
So to solve this, the idea was to artificially postpone puberty until the person is old enough that they can make a mature, informed decision about the subject.
Of course that logic has its own weak points - e.g. that puberty itself may play a major role in the person finding their identity. So postponing it until the person has settled into an identity would be sort of like trying to outrun your shadow. I'm not sure what the status of science in that regard is though.