There are regulations, born from JARs but incorporated into every airline ops manual that I am aware of, which lay down a specific set of conditions whereby you can arrive at your destination airport with less than (ii) and (iii) on board; but never less than (iii).
In other words, allowing you to burn your alternate fuel but still land with (what we also call) final reserve fuel on board.
We can burn the 'Contingency' fuel (i) at the holding point if we want, or by taking an extended routing, or by using lots of Anti-ice etc etc, as long as we can land at my destination with (ii) + (iii) on board (or just (iii), under the circumstances mentioned above).
Therefore, I believe (and my company SOPs agree) a much useful application of the USING RSV FUEL message in the B737 FMC (resulting from that input of that RSV FUEL figure you mention), is to alert us when we are no longer predicted to land with (ii) + (iii) on board; i.e. to prompt us to check whether the conditions are OK for us to continue.
Contingency fuel (i) = to allow for contingencies en-route; as Capt S&L says, it could already be gone.