One of the features for CC3+ symbol catalogs is the ability to arrange these into groups, and then set this group to place random symbols from the group, or apply random transformations to them, like rotations or minor scaling to give variety to otherwise identical symbols.
But, what if you are making a particular map, and you need some other kind of grouping? For example, when placing trees you want to randomly place Decid, Pine and Jungle trees among each other? There are no predefined group like this in most symbol catalogs. Well, for that you can quite easily set up your own personal random collection just for the current map (or you can save it into a symbol catalog if you want it available later).
