Parentheses are used to denote several polyatomic ions is a compound.
Note– This question is actually a chemistry question rather than an English question.
Although both chemists and writers use parentheses in their work, their functions are rather unrelated in those respective fields
However, when you use parentheses in a chemical compound, it is meant to distinguish the number of polyatomic ions from the number of atoms in a given polyatomic ion.
For example, calcium nitrate is written as ##Ca(NO_3)_2## to show that there are two nitrate ions for every one calcium ion. If it were written like this ##CaNO_32## it would mean that there would be 32 oxygens which would be ridiculous and misleading.
In short, parentheses in chemistry mean to make clear when a subscript is describing an atom or an ion.
Hope this helps!