Some people would like the command that ticks an article (!) to go to the next article. Others would like it to go to the next unread article. Yet others would like it to stay on the current article. And even though I haven't heard of anybody wanting it to go to the previous (unread) article, I'm sure there are people that want that as well.
Multiply these five behaviors with five different marking commands, and you get a potentially complex set of variable to control what each command should do.
To sidestep that mess, Gnus provides commands that do all these different things. They can be found on the M M map in the summary buffer. Type M M C-h to see them all—there are too many of them to list in this manual.
While you can use these commands directly, most users would prefer altering the summary mode keymap. For instance, if you would like the ! command to go to the next article instead of the next unread article, you could say something like:
(add-hook 'gnus-summary-mode-hook 'my-alter-summary-map) (defun my-alter-summary-map () (local-set-key "!" 'gnus-summary-put-mark-as-ticked-next))
(defun my-alter-summary-map () (local-set-key "!" "MM!n"))