Rman scenerios

1-) When backup is online and which is not consistent ,when you restored yesterday backup then yours datafile is not consistent i.e the CKPT SCN is not sync with controlfile ,to make yours datafile sync itself and with controlfile you require recovery.

2-) Oracle first check controlfile checkpoint SCN number ,if checkpoint SCN in the datafile header earlier then controlfile checkpoint SCN then oracle need redo,recovery is overall the process of applying redo to data files.In more depth recovery will synchronize the checkpoint SCN in the data files header to checkpoint SCN in control file Or in broader term we can say goal of recovery is to make sure the CKPT SCN in each datafile header matches the CKPT SCN in the control file.

3-) When you restore old control file which doesn't know where the redo chain ends forces open resetlogs.