It can depend on the exact scenario, but if you omit the call using the alias name in your modification, it is really no different than overwriting the method.
Take a look at the example above.
The first call to in the modified method is to create the sprite. This happens BEFORE the original "main" method is called. We then call the original method. Processing is stuck here while the scene is active, because of the loop that is placed in scenes, so the any code after that call to the original is made as the scene exits. This is where we dispose the sprite.
Circumstances where the method returns a value are sometimes handled differently. Sometimes you need to see what the original method returns, and then process that data further. For example, let's say we want to modify Game_Actor so that if the name was "Tom" we return "Jerry" instead. We want the normal name returned in all other circumstances, only when the name is "Tom". You might make something like this.
class Game_Actor
alias change_name name
def name
string = change_name # Stores original return value in variable "string"
if string == 'Tom'
return 'Jerry' # Returns 'Jerry' if name was 'Tom"
else # If not 'Tom'
return name # Returns the original name
end
end