For example, if you are sending any ERC20 tokens to 0X ethereum address, they are not retrievable forever. so you lost them. there is no way to get them back. similarly, if you are sending them to an invalid address(which is not compatible with it at all), the coins/tokens will be lost. Coin/Token developers may use the first approach as per my knowledge.
Purpose: To reduce the inflation burning is the common thing to follow provided they have huge amount of coins/tokens available
When the swapping is done: Sometimes, devs create a new coin/token/project and use the old/existing coins/tokens. People who possess old coins/tokens can get the newly created coins/tokens by this method. If there are some old tokens/coins are left out, dev can burn them to reduce the inflation or to quote the exact available coins or to clean up the old ones all the way.