The main reason why this happens is in order to protect the testes. If it would not descend, there is a big chance that it will get damaged. This is not something that anyone would like to happen. The testes usually descend on their own due to different influences that are hormonal and mechanical by nature. The descent of the testes will actually begin from the 8th week to the 15th week.
The process will become even faster by the time that the 26th week rolls around. If the testes fail to descend, there is a big chance that the baby will have fertility problems when he grows up. If not, there may be other medical conditions that may arise.