This checks to see how many notifications should be sent out, and also how many e-mails will be sent out at a single run of the background task. So if you have 4 or less people asking to be notified, it will do it immediately. If greater, it will be queued to a background task that runs with your task jobs. Setting this too low, as it is implemented now, could have undesirable consequences as instead of sending out 30 notifications per task run, it would only do five. If you set this too low, it could take quite a bit of time to process those e-mails as those notifications from all threads are going to get queued.
I would not set this option today and leave it at the default (or set it higher if your setup can handle it).
The issue is that the time it takes to send out those notifications for the first four emails may cause hanging as the communication happens with your email solution. This doesn't matter if it's a task happening in the background, but when it's immediate, it impacts the user experience by causing a delay in the time from clicking submit to the redirect to view the thread.
I'd like to see a QUEUE_ALL_NOTIFICATIONS constant or similar to turn off the immediate e-mail bit and send everything to queue.