Blocks containing only the coinbase transaction are referred to as "empty blocks".
Miners will mine on an "empty" block (containing only one transaction to themselves) while assembling the new block of transactions they intend to mine. Occasionally they find a hash solution in that brief amount of time and therefore broadcast the empty block to collect the block subsidy.
This only makes sense because the subsidy is currently much higher than the sum of all transaction fees.
Although a miner could be intentionally mining empty blocks to try and delay transaction confirmations, it is unlikely as they are simply depriving themselves of the fees and passing that money on to the miner of the next block.
