FD, your argument is flawed – “You can land safely if ALD (actual landing distance) is less than LDA - in flight calculation requirement.”
The statement assumes that have a specific definition of ‘safe’ – the process of risk management (acceptable to the authority, etc, etc, and after the event), and also that you can calculate ALD with sufficient accuracy.
Using the Boeing format for ALD, it is generally recognised that the baseline data is not consistently achievable in normal operations, thus a factor is required to provide a starting point for corrections.
The alternative OLD, provides a much more realistically achievable baseline, FOLD provides a factor, normally 15%, so that this value is a better starting point for corrections and additions.
JM, c100, it would be incorrect to consider reverse as a benefit. Certificated data does not normally include credit for reverse, only if it mests the requirements for reliability. Comparing reversers with other systems, then they are not that reliable, and don’t forget that the engine can also fail.
The 15% in the SAFO might only increase ‘actual’ data to a value that can be expected to be achieved in normal operations, i.e. similar to the distances in OLD. FOLD adds a further 15% minimum with an expectation that the crew will add more according to the landing conditons.