A foot stepping forward from a line: to go. Classical Chinese borrowed it as the all-purpose connector 'of' and pronoun 'it', and it never left — modern idioms are full of it.
百分之十 = 10 percent ('ten of a hundred'), 三分之一 = one third. Elegant writing loves it: 之前 before, 之后 after, 之间 between. It is the classical 的 — same job, older suit.
A FOOTPRINT linking two places — the classical OF.
Every Chinese percentage contains 之: 百分之五十 'fifty of one hundred'. You cannot say 50% without this 3-stroke fossil.