The score is counted automatically when a contact makes an activity or meets a rule. Re-calculating is needed only if you change the rules. In this case you can recalculate the score manually in CMS Desk -> On-line marketing -> Scoring -> edit -> Recalculate. The scheduled task is helpful only if editors often change the rules and could forget to recalculate the score.
Please be aware that the scheduled task throws away the current score and counts the new values on the fly, so you can see temporary results there while the task is processed. Also all the Limit notification e-mails are sent again if the scheduled task is executed.
See also: Managing scores