Even discord and slack I don't want browser notifications. Inside their own tab, I can deal with ui elements. There's no need nor no want to have a page alert outside of the web page itself.
It can be useful when you get mentioned, like for example, a user has pinged the moderator role where a quick reaction will be necessary. Desktop notifications can significantly reduce reaction time to important events (though, assuming you do the sane and set Discord to only notify you on mentions that aren't @here or @everyone)
Well, if you don't know me well enough to have my phone number, you certainly shouldn't have the privilege of making my computer beep at me, whenever the hell you feel like it. But, that's me. Maybe you like people interrupting you. (Or maybe you like interrupting people because you think you are too important to wait en queue.)
Study websites reminding me that it is time for my next study session are great for establishing the habit of doing my studies.
When I was single I would have had time for 3 of them - total in my life. I think systems should automatically block all requests for notifications if you already have 3 since it is not possible for more than that to be useful. (the exact number is debatable, but 3 is close). Note that I said systems - I want firefox, chrome, ie, safari to work together across all the computers I have to keep the number down - for security reasons I understand they cannot but that would be ideal.