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In "Mastering Bitcoin", the author notes (source):
The maximum number of coins mined is the upper limit of possible mining rewards for bitcoin. In practice, a miner may intentionally mine a block taking less than the full reward. Such blocks have already been mined and more may be mined in the future resulting in a lower total issuance of the currency.
I understand that a possible answer is "just because" but I'm wondering if there are other reasons behind such an action.
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Likely you're correct, it's not useful in practice. People could, however, use value destruction as an costly signal.
Voting example; I run a poll of mining pools; number of Satoshis less than the full block reward, up to 5, is the vote for which of 5 proposed algorithms to run in the future. Once a majority of blocks over some time period vote for option X, everyone agrees to switch protocols. (This could allow identityless voting by miners.)
Commitment example; A miner can "promise" something, and commit the value destroyed as a signal that they are willing to do it. If a miner wants to extort the network, such commitment could be effective in showing that they are serious.
2(These examples / ideas are intended as a thought piece. It's unlikely they are ever the best way to do the things mentioned.) – David Manheim – 2015-12-30T22:06:58.470
Pieter Wuille mentioned some more blocks that took less than the full reward on Will there be 21 million bitcoins eventually?.
– Murch – 2016-03-11T10:25:47.443