Benefits of using EventHandler - vb.net

Question

I'm having trouble wrapping my head around Events and their Handlers in general. I was working from some sample code that used them, and I can't understand why use an event rather than simply using a sub. I'm absolutely sure I'm missing the bigger picture here.
Abbreviated code example:
Public Class MenuEntry
Public Event selected as EventHandler(of EventArgs)
Protected Friend Overridable Sub onSelectEntry(e as EventArgs)
RaiseEvent selected(Me, e)
End Sub
End Class
Public Class Menu
Private menuSelect as New inputAction(Keys.Enter)
Private menuEntry as New List(of MenuEntry)
'keeps track of which menu item we're currently on
Private _selectedEntry as Integer
Public Sub Update()
If menuSelect.evaluate Then
onSelectEntry(_selectedEntry)
End If
Protected Overridable Sub onSelectEntry(ByVal entryIndex as Integer)
menuEntry(entryIndex).onSelectEntry(New EventArgs())
End Sub
End Sub
End Class
Public Class OptionsMenu
Inherits Menu
Private arbitraryOne as Integer
Private arbitraryTwo as Integer
Public Sub New()
Dim entryOne as New MenuEntry(String)
Dim entryTwo as New MenuEntry(String)
AddHandler entryOne.selected, AddressOf entryOneSelected
AddHandler entryTwo.selected, AddressOf entryTwoSelected
MenuEntry.add(entryOne)
MenuEntry.add(entryTwo)
End Sub
Private Sub entryOneSelected(ByVal entryIndex as Integer)
arbitraryOne += 1
End Sub
Private Sub entryTwoSelected(ByVal entryIndex as Integer)
arbitraryTwo += 1
End Sub
End Class
And I was right, I was missing the bigger picture. Writing out all the code in the same place helped me to see exactly what was going on. Hopefully I'm correct:
The Event allows a class to say 'Do something when this happens' in a very ambiguous way, leaving the class which created the Object to define a Handler; what that action should be. That Handler can, and very likely will, be unique to each instance of the class.
It seems to me that this would likely be achievable (on a basic level) through indexing and enumeration, but that would get messy and become a lot of code to write rather quickly. This is probably a much more flexible and extensible way of handling things.
I'm going to post this anyway, in the hopes that I'll get someone to tell me whether I am correct in my observations or totally off base, and that it helps someone else who is having trouble with this concept as they dip their toes into OOP and event driven objects.

Answer 1

Being able to have arbitrary code being run when the event is raised is an important aspect. But there's a much bigger benefit, it strongly reduces the coupling between classes. Note that your MenuEntry class has no reference to the Menu class at all. You can completely redesign the Menu class and not have a need to make any changes at all in the MenuEntry class. That makes code highly composable.
The technical term is the Observer Pattern. The Gang of Four book is essential reading for programmers.

Answer 2

Yes, your observations are correct. Events are used to let all interested parties know that something has occurred and, if they are interested, they can then perform some additional actions that are not necessarily intrinsic to the class that raised the event.