Issue trackers notify you about everything, which in practice means about nothing: the update that needed you arrives in the same pile as the label someone renamed.
Routing a few tracker events to your phone, and only those, gives the important ones a channel of their own. An urgent bug filed by support or an SLA about to breach is worth an interruption; a typo fix in a description is not.
Most useful for small teams, or filtered to a team or project that files few but urgent ones.
Where the question you are blocking usually is.
The tracker already knows the clock is running out. This makes sure a person does.
For the handful of issues you are waiting on, rather than every update in the workspace.
Each of these sends the events above. Setup is the same for all of them.