why would somebody do it? two reasons i guess, depending on how extreme you want to get. the less extreme version of silicone injections do not lump up like that. a doctor can inject 100 or 200 ml of silicone into your penis or scrotum and the result is pretty good. it simply makes you fatter, permanently. it's supposed to feel great. i have seen guys with minor injections and the results look decent.
the more extreme version is putting 500 ml or even a full liter of silicone into your parts which makes you have big cartoon sized genitals. you can't penetrate anymore but some guys are into getting as big as possible. the guy in the video is somewhere in between. more than a minor injection, but less than a freak.
the injections are permanent. when the silicone goes into your body, it's not coming back out, ever. that's why most guys do saline injections first.