Before there were seminaries, and even before there were universities, it was the bishop himself who was responsible for a candidate's training. That was the purest and most direct form of priestly formation. From the bishop a candidate learned holy doctrine. From the bishop he learned how to handle the holy things. And from the bishop he learned holy living. By contrast, our seminaries barely manage to teach the first of these three. (To be sure, I am a fan of the concept and tradition of the seminary-I just wish modern seminaries would pay more attention to their genuine responsibilities, and clean their house of the distractions of their contrived responsibilities.)
In the course of time, priestly formation began to take place in other ways, such as monastic training, cathedral schools, universities, or the modern seminary. And yet, even when a man is prepared for churchly ministry in one of these other ways, it is still the bishop who is responsible for his training. The bishop is merely entrusting, or delegating, certain aspects, or even all, of the training to this or that specialist, or to a school, etc.
The relationship between the parent and the primary & secondary schools is perhaps an apt analogy. It is the parent who is called upon to raise and train the child. The school, the teachers, even a private tutor, these are only taking on certain aspects of this training/raising on behalf of the parent. Likewise, the bishop is the spiritual father of a church, and he is called upon to teach and train. The seminary merely takes on this role on his behalf.
In terms of the LC-MS culture, this is not a readily apparent truth. For we have a less than traditional concept of episcopal authority. Seminarists on their year of training in the parish ("vicarage") often refer to the vicarage supervisor as their "bishop." There is truth in this, and I don't discount it. Yet it would be good for the seminarist to recognize that his bishop is still the one who sent him to seminary, which in one sense is his home pastor, and in another sense is his home district president.
Interestingly, LC-MS custom seems to implicitly recognize this, since ordination is reserved for the district president, who may delegate it to another pastor.