The top is 水 (Water)
The bottom is 皿 （Pot)
Both characters shared the same shell bone script, or we can see that the original meaning of benefit is overflow: You have more than you need.
Because the creation of characters is not easy, at the beginning there were not many characters, so it was common for character to have multiple meanings, normally related meanings.
The boiling water is overflow the pot if the pot is full of water, just like the water overflow the dam
The small seal script for the character of Benefit is
The top is 水 （Water) in horizontal position, which means the water flow in horizontally, not vertically. The bottom is Pot or Cup, like a cup of overflowing beer.
Small seal script was created during 221 BC to 200 BC when people created another character for Overflow (you will see later in this post).
The clerical script for the character is
It looks like overflow: too much to hold on, or more than we need, which is similar to benefit, or good for, which is Benefit.
In order to avoid confusion, ancient Chinese created another character in big seal script for Overflow back to 3000 years ago right after shell bone script time, and long before the small seal script was created
The big seal script for Overflow is
There is a Water on the left, and the right part is character Benefit. The water on the left indicated that the character is related to water, so it is overflow, not benefit.
The small seal script for Overflow is
Similarly, the water on left to indicate Overflow, not Benefit.
The clerical script for Overflow is
The two characters are from the same shell bone script, but evolved to different meanings. To distinguish them, ancient Chinese added a water to the character for Overflow, and original character became Benefit.
The Pinyin for Benefit (益）is Yi4, and the Pinyin for Overflow (溢）is Yi4.
Both are pronounced exactly same which is very confusing to beginner of Chinese learning students. But it is very clear to see the difference of the two in writing. So if you only learn oral Chinese, it is difficult soon after you learnt some words, you will be confused by too many homophones and 4 tones.