When the You configure the notification in the Alert categories the users would get a notification when every the particular alert is generated , where as the when you configure notification in the backup job then the recepient would get a notification with the status of that particular job and it can also include the job log.

So when you enable receipent in the job properties he would only get alerts for the that particular job where is you configure notification in the alert catalogiers he would get notification for all the jobs as wll as notification for the alerts which are not related to backup job.