What is an Instance recovery? Why does it have to be done if we shutdown abort?

What is an Instance recovery?
Why does it have to be done if we shutdown abort?

Instance recovery is the process of applying records in the online redo log to data files to reconstruct changes made after the most recent checkpoint.
Instance recovery occurs automatically when an administrator attempts to open a database that was previously shut down inconsistently.
So you don't have to do anything is this case.
Oracle manages it by itself