Uses of base::Callback instead of a ButtonListener removes the need to check who sent the event (whenever the listener can receive >1 events).

Example pseudo/C++ instead of base::Callback:

AddChildView(new Button([this]() { DoEvent1(); });

vs.

enum EventTags {
  kEvent1,
};

...

views::Button* button = new Button(this);
button->set_tag(kEvent1);
AddChildView(button);

...

OnClicked(sender, ...) {
if (sender->tag() == kEvent1) {
  DoEvent1();
} else if (...) {
  DoEvent2();
} else {
  DoEvent3();
}

Which is harder to follow/easier to get wrong (if you in this case added Event4, DoEvent3() would be triggered for that button press).