This allows for much finer control of the transaction fees per kilobyte as it prevent small transactions using a fee that is more appropriate for one that is of a kilobyte.
This also allows controlling the fee per kilobyte over rpc such that:
bitcoin-cli settxfee bitcoin-cli estimatefee 2``
would make sense, while currently it grossly fails often by a factor of x3
This fixes issue #6479, and has minimal impact as dynamic fees are currently the default. This default can be overriden in the qt wallet, although I have trouble imagining where that is useful. Possibly the entire concept of fPayAtLeastCustomFee can be removed in a later version.