Electing a slave as new master and aligning the other slaves to the new master
In a simple MySQL Replication set-up you have high-availability (HA) on the read side (r). But for the master which covers all the writes (w) and the time critical read (rt) there is no HA implemented. For some situations this can be OK. For example if you have rarely writes or if you can wait until a new Master is set up.
But in other cases you need a fast fail-over to a new master.