No-kill shelters decrease the amount of stray animals. They take in stray or unwanted pets, so they are efficient in reducing the number of homeless animals. Typically, no-kill shelters spay/neuter their animals, this is helpful because, in the case of abandonment or if the animal gets loose, they won't reproduce and add to the already-overwhelming pet population. If there is an overflow of pets, the shelter reaches out to find volunteers to foster some of the animals. There is no need to euthanize a healthy animal.
Often, in kill shelters, animals are euthanized simply because the shelter can't house anymore pets. Although kill shelters are efficient in controlling the pet population, they are often seen as inhumane. When animals are brought there, they are usually given a certain amount of time to be adopted before they are put to sleep. That means that if your favorite pet gets loose and ends up in that shelter, you only have about a week to find them. If they aren't found in time, they will never be found. This can truely be heart-breaking. Additionally, kill shelters don't always fix their animals. Therefore, they aren't productive in reducing the pet population.