It is not needed for displaying the notification but specifically targeting users. You would usually call an FCM function to get a token, which can be used to specifically target that user and is stored in the database. To send a notification to your customised target audience based on user-generated events, it becomes essential to use web-service that does it for you.
For example- Let's say you wish to send a notification to user A when user B sends him a message. In this scenario, you can call another FCM fn, using the token user A had generated. Of course you can use the web dashboard to send the notifications, but in this case, you would not be able to send event-based notification reliability and in real-time. Other services also work in a similar manner.