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.