There could be many reasons, but the most common and important reason is to free them from the suffering they experience when they are killed. When you physically release an animal of course you free that animal from being killed, but at the same time you ignore the conditions that will result in killing the next one.
As some people will rescue animals from being killed at shelters, they will still eat meat at their next meal.
So the wise way to release a captured animal is to be a vegetarian. This is especially true now when it is common for people to eat many different kinds of animals.
If there are people who eat meat then there will certainly be those people who kill for them. If everyone quit eating meat, fish, chickens or those living beings, then naturally the killing will stop. That is why I think the best way to release the captured animals from the prospect of being killed for food is for everyone to fully become vegetarian.
Research has shown that vegetables are healthier then meat for our body. Of course that is a bonus for us. But those who practice Buddhism need to generate our loving kindness and compassion. For those who practice Buddhism being a vegetarian should not be based on the health benefits that accrue to the vegetarian.
For the practitioner of Buddhism the bottom line is to cultivate our compassion towards sentient beings. It is true that this is very hard for many people to do. Because our bodies get used to this kind of non-vegetarian diet it may be very difficult to become a vegetarian immediately. However, if we are willing to change our diet to a vegetarian one, we can change our lives, but it may take a longer time.
If you want to practice eating a vegetarian diet in order to become a vegetarian, you can choose to be a vegetarian once a week or twice a week with the right and genuine motivation. Gradually your system will accept the new diet and can eventually become fully vegetarian.
From the Buddhist point of view, the proper motivation is the most important thing. So if our motivation is releasing those suffering beings, once we start to be vegetarian we are releasing these beings from suffering every day. When we have the right and wholesome motivation it most certainly will lead to wholesome rewards and retribution.