You would normally keep important groups on high levels, but that scheme is somewhat restrictive. Don’t you wish you could have Gnus sort the group buffer according to how often you read groups, perhaps? Within reason?
This is what group score is for. You can have Gnus assign a score to each group through the mechanism described below. You can then sort the group buffer based on this score. Alternatively, you can sort on score and then level. (Taken together, the level and the score is called the rank of the group. A group that is on level 4 and has a score of 1 has a higher rank than a group on level 5 that has a score of 300. (The level is the most significant part and the score is the least significant part.))
If you want groups you read often to get higher scores than groups you
read seldom you can add the gnus-summary-bubble-group function to
the gnus-summary-exit-hook hook. This will result (after
sorting) in a bubbling sort of action. If you want to see that in
action after each summary exit, you can add
gnus-group-sort-groups-by-rank or
gnus-group-sort-groups-by-score to the same hook, but that will
slow things down somewhat.