The notifications section is optional when creating a campaign, but it is useful for receiving a software notification once certain parameters are met after a campaign is sent.
The options provided here are in this section are the number of opens or the number of clicks as a total or for a particular recipient.
Most email marketing programs no longer put a lot of weight against 'Opens' and both Google and Apple are 'auto' opening email to pre-cache images anyway. What is more important is tracking 'clicks' which is now considered a better indication of engagement with the content.
For example - if a particular user clicks multiple times, that may be an indication of high intent and worth following up.