I understand both are vulgar male forms of eat, but why these two kanji are basically the same? What means that 口 in there？
The earliest appearances of 喰 in dictionaries are, as @変幻出没 said, those in 龍龕手鏡（龍龕手鑑） or 新撰字鏡, but their definitions are not much same as today's. The meaning they suggested is like "dine", "dinner" or "have a meal" and likely to be a variant of 飧.
In more recent usages 喰 explicitly represents くう and くらう, contrary to 食 can also be read たべる or はむ. I'm not sure if the kanji was recycled or reinvented, but it certainly had some usefulness because pre-modern orthography rarely used okurigana, making it difficult to distinguish certain homographs. In this sense, you can also say that 喰 has already lost its reason for existence, but it's still in use, especially when you want more "bite and booze" feeling or violent "consuming, devouring" implications.