Parentheses are used to denote several polyatomic ions is a compound.
Note– This question is actually a chemistry question rather than an English question.
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!