There are many reasons legal terms have a certain vagueness in them. It’s future proofing, for a start: the law must be flexible as societal norms change. Sometimes, it’s unintentional, which leads to some interesting applications. Part 5 of the Serious Crime Act 2015 is a really good example of a law designed to stop one thing and ended up being quite broadly applicable. Other times, a term is left vague because nobody can come up with a decent definition, and that’s the problem the UN will potentially face.
A good legal definition is narrow. It’s specific in its boundaries: it creates a class of thing that is easily applied to individual cases, accommodates change, and doesn’t exclude things understood to fit the definition. It does all this, but it also needs to exclude things understood to not fit the definition. In an international treaty, it needs to be sensitive to each signatory’s practises at the time, but also create a premise for conformity: the idea is to make signatories change how they do things. Trump wants to define what a woman is legally so most of the world has to conform to it.