Every month the diocese sends out the Diocesan Memorandum giving updates about important things around the diocese about which we should be aware. You might be surprised how often we receive warnings about persons impersonating priests who might be in the area.
The Church is rather particular about appropriate persons doing the right things in the right place. Sometimes it might seem that it is overly concerned. But when you are trying to keep things clear, easy, licit, and valid for over a billion people it serves to be particular about such things.
Just for example, let us say that you want to get married and need a copy of your baptismal certificate. Where is it? Why at the parish at which you were baptized of course. It will always be there. It does not matter where your actual home parish is. If you uncle, a priest, baptized you at St. Soandso, you baptismal certificate and all subsequent sacramental information about you (confirmation, marriage, holy orders, public perpetual vows of chastity etc) will all be shipped to this parish even if your actual parish is St. Whatsoever. The rule is the rule is the rule and everybody will always know where to look for your certificate.
This also helps when trying to determine of a sacrament is both licit and valid. Is the priest actually a priest? If he is from outside the diocese he will need a letter of suitability from his bishop. Within the diocese he will need permission from the local pastor to perform many sacraments. How does this help? First it helps us avoid people who are impersonating priests. (Again, you would be surprised.) It helps keep records straight. And it safeguards, for those to whom it matters, against having a person perform your sacrament who may no longer have faculties to do them. Many a man who left active ministries or who has been removed has declared himself still active and goes about performing ceremonies just the same. This is the case with the rather famous Rent-a-Priests. A person who uses them may consider themselves married or confirmed or even ordained, but it is not in any way recognized within the Church. This also goes for “Catholic Churches” no longer in union with Rome even of the priest was ordained validly. He has no faculties and no jurisdiction.
There is an exception to all this of course. At the point of death, any priest, even if he is outside of his jurisdiction, even if he left the priesthood, even if he was kicked out may perform emergency sacraments (confession, anointing) and it is both valid and licit.