bitcoind without txindex=1
I am wonder why this works.


Because all options have the default value.


  The question is what txindex=1 do.


I can't find this option in the documentation

From the documentation


  -txindex Maintain a full transaction index, used by the getrawtransaction rpc
  call (default: 0)


If you're using Bitcoin Core just for your own personal use, you probably don't need the -txindex=1 option. But, if you're using Bitcoin core for development or blockchain analysis of some sort, you will need to set -txindex=1 to be able to get transactions data for any transaction in the blockchain.