[DominikD]

In female cats and dogs neutering eliminates the risk of pyomyxia. The risk in females that have never gave birth is extremely high and consequences of delayed action when it happens are fatal (not to mention that cost of surgery may be prohibitive for many; it's orders of magnitude more expensive than neutering).

For male cats neutering reduces territorial spraying (urinating in order to mark your space; the smell of the urine is also much stronger if ca isn't neutered).