Hopefully it helped.
In case you want a little more, here’s an analogy that will hopefully clarify things.
A bunch of jelly beans. This is like your loose, ungrouped geometry.
The jelly beans in a jar. This is like putting your loose geometry into a group.
If you want to make changes to the jelly beans in the jar, you have to open the jar. (Open the group for editing.)
If you are going to use a tag, you add it to the jar, not to the jelly beans. The group is given the tag/layer) not the edges and faces inside.
When you want to make changes to the contents of the jar, it doesn’t matter what the tag is on the outside. You simply open the container and make the changes.
Maybe instead of jelly beans, think of strawberry jam as the loose geometry. It’ll stick to everything unless you put it in a jar.