= is a comparison AND a set operator, but:= is just a set operator.
If 7 = 7 Then ...
Dim myInt As Integer = 7
Say you have a custom object called SuperList whose constructor takes a variable called initialCount, then you can do things like:
Dim myList As New SuperList(initialCount:=10)
It's just sometimes easier to read a constructor when you know what values you're setting, especially if you have a constructor like
SomeConstructor(12, 432, True, False, 32)
It makes more sense to see
SomeConstructor(monthsInYear:=12, daysInYear:=432, leapYears:True, leapDays:=False, daysInMonth:=32)
There's probably more but this is what I got off the top of my head.