general question about groups

[.Martin.]

I'm getting more and more familiar with gnus. The thing is that
regardless of a newsgroup reader, I'm still very new to newsgroups in general.
The more I read into gnus tutorials the more I come across concepts
like: group level, scoring, marking, ranking.
I'm probably missing something here, but what's their practical usage?
Do you score/rank/mark/set level to groups? If so, how does it help you
later?

thank you in advance

-- 
regards

.Martin.