>>What's the point?
What a daft question.
If you don't care, don't piss on those who do.
>>How does one decide which events should be tributted?
We each decide for ourselves what things we wish to pay tribute to based on how events affected us.
Sometimes, events happen which mean that other people want to pay tribute to things more than others.
>>How long should an event remain commemorated?
We still commemorate the 5th of November (401 years) Armistice Day (88 years) & probably others.