I'm sure there are a number of legitimate uses but as of now, every craptastic "news" website I visit now wants to pester the hell out of me with notifications when they post new clickbait.
It should always be opt in, but it is a useful thing for many use cases.
In Firefox, just open "about:config", search for dom.webnotifications.enabled, then double-click it.
If using other browsers, don't :)
(PS: I like desktop notifications for Slack, but since I'd prefer not to use Slack in the first place, I'm not sure I count)
Go to about:config, search for dom.webnotifications.enabled, set it to false.
I'd be OK with a small icon in the address bar to indicate push notifications are available on a site but even one pop up asking for permission is too many.