If a job is particularly long running then it often doesn't make sense to want to schedule just one thing in the queue, especially if you are using multiple workers to process jobs. Groups allow large tasks to be broken up into smaller ones. A common pattern is to have a job executed at the beginning of a group that makes lots of small jobs that do the main body of work and also may add in a final job to do any clean up or bookkeeping needed after the task is completed.
Once any job in a group has been executed it is no longer possible to create a new group with the same name, nor to add more jobs to the existing group. Group names should be chosen and used accordingly.