From phpBB Development Wiki
What are events?
An event is a hook location, also once known as a ledge. When an extension wishes to hook its functionality into the phpBB core at a certain point, it can do so only at available hook locations, or events. For more information, see the wiki article for listeners.
Why do we need events?
They link the identifiers to the point where the code needs modifying and as such they are vital.