In today’s Parsha, we read about the subject of מום, blemishes, that could disqualify a Kohein from serving in the Beit Hamikdash and for an animal not being allowed to be offered as a sacrifice.
The Rambam lists ninety different types of מום for an animal and those same ninety plus fifty more for a Kohein.
Although the Kohein and animal are disqualified for the עבודה, the Kohein with the מום can still eat sanctified foods, and the animal with the מום can still be eaten as a Kosher animal.
The case of the בכור, first born animal, still has קדושה even today. Since this first born cannot be offered in the Beit Hamikdash, we solve the problem by sending the animal to pasture. When it develops a מום, it is allowed to be eaten.
This was a question in the Gemara as to whether a Kohein is believed to say that an animal had a מום. If it did, the Kohein would get it for personal use. If not, it was offered as a קרבן.