Older sperm can lose their ability to swim well. A good masturbatory flush guarantees fresh, robust sperm for mating. Storing seminal fluids for long periods can also cause prostate congestion, which in turn can lead to urinary and ejaculatory pain. This is especially true in females where strong PC muscles are practically the sole factor in whether labor is easy and fast or long and difficult. Females masturbating regularly with multiple orgasms would develop strong PC muscles and should have easier labor.
Masturbation is also an ever-renewable health resource.
In fact, the people who start the earliest and do it the most often are the ones who do it longest into old age. Another potential concern is reliance on masturbation for sexual pleasure to the exclusion of intimacy with another person. But in such cases, masturbation is probably a symptom, not a cause, of larger psychological barriers to intimacy. Most people want relationships. But if you're scared of them, you might stick with masturbation exclusively.