Burning unsold tokens in my own understanding will reduce the circulation supply and therefore the price value would rise.
There are some instances and incidents that in the past, some unsold tokens are being hacked and developers have no idea whose the responsible for the sudden token distribution from the main wallet.
Oyster PRL is burning 10M tokens from developers' wallet to reduce the maximum supply and to maximize the possibility of token price value will soon rise and every holders could benefit.
Exactly, the coins in circulation will be reduced making the price of the remaining coins skyrocketed. And that what we investors like, coins that is really good for long term hold because the price will surely increase because of its scarcity and limited supply. So i think its a good thing to do with some ICO tokens.
Also burning tokens will ensure that there will no sudden dumping because it doesn't look good at the eyes of the investors. And besides, who are going to hold the unused tokens? the dev? again this will not sit very well with investors because at the back of our minds they can suddenly dump it anytime causing the price to plummet.