Why is it important for the testes in pigs and other mammals to completely descend into scrotum?
The production of sperm requires a lower temperature than body temperature (in mammals) therefore it is necessary for the testies to be removed from the general mass of the body.
To be clear on the terms: The scotum is the sack-like pouch beneath the penis. Inside the scrotum lie two testicles. From your questions, I'm guessing that you mean testicles and not "scrotum". If you mean testicles, and not scrotum, the condition you are describing is most likely cryptorchism. This condition arises when one (or both) of the testicles do not descend from the abdomen within the first year or so after birth. The testicles…
For sperm production, the testes need to be slightly cooler than the normal body temperature and to achieve this they descend into the scrotum which is outside the body core. The testes are located outside the body in the scrotum, because sperm production requires a cooler temperature then the internal body temperature of animals.