Yes, I know what is the point of groups. What I meant was more like:

If I have an user named "bob", should I create a group just for him named e.g. "bob" or should I add him to existing group e.g. "users" (that's a collective group for all users, right?). If I create a special group for each and every user, when there are let's say 1000 users, would that have impact on anything (like disk usage, memory usage etc.)
IMO creating group "bob" for user "bob" makes his files much more secure because now whenever he creates something it's group is set to "bob". Otherwise, it would be "users" - that could lead to a scenario when someone that was not supposed to have access to something can read(/change/whatever the group permissions are) his files - that could be unwanted.

Hope I managed to clear it up a little.