It has been shown in some studies that frequent ejaculations no matter how they are achieved, may reduce ones risk for prostrate cancer. However, abstaining from masturbation (ejaculating) doesn't necessarily mean that one will get prostrate cancer.
It is true that the body can and does reabsorb unspent semen, but it is also very likely that one would have nocturnal emissions (wet dreams) as a means to clear out unspent semen.
Nocturnal emissions can occur all throughout ones life but they are more prevalent among younger males, especially teens.
When those websites say a male "needs" to ejaculate, it's more or a reference to the fact that males are made to do so, it is what we do and doing it is as natural as eating, breathing and going to the bathroom. It can contribute to better health and well being by reducing the aforementioned risk for cancer as well as being a safe and effective way to relieve pent up sexual tension and stress in the absence of partnered sex, it also allows the body to create and have ready a fresh - healthy supply of semen as well as keeping your reproductive organs exercised.
As long as you do not let masturbation interfere with your normal daily activities, social interactions and responsibilities there is nothing harmful, wrong, bad or immoral about it. However, it is not "required" that you do it if that is your wish, it's just that there are some benefits to doing so.
In the end, it's OK to masturbate in moderation if you choose to do so, but it's also OK to abstain.