When injecting a transaction from a tz1 account to another one,
tezos-client will add 100 to the estimated gas. We know that such operation consumes 1427 gas, why increasing it by default? What could go wrong otherwise?
The client always applies the same gas safety margin of 100 gas units and it is indeed useless for transactions to implicit accounts. Don't hesitate to open a feature request on https://gitlab.com/tezos/tezos if you would like the client to be smarter about this.
We know that such operation consumes 1427 gas
This is not true. During the Delphi protocol upgrade, gas costs were significantly reduced. You can now send to a TZ1 address for ~500 gas.
Estimating fees for each request ensures that you benefit from any changes in the latest protocol, and/or any new costs are taken into account. 100 is added to estimations as a safety buffer to help reduce the chances of an estimation error preventing a transaction from succeeding.