As has been covered above,
It cannot be planned to be used under normal circumstances, especially when enroute etc.
Adequate Variable reserves or margin fuel are meant to allow for any forseeable enroute contingencies, stronger than forecast winds, unforecast adverse weather, systems failures, eg depressurizing and forced to fly at much reduced levels etc.
Yes it can be used once you are overhead/ arrive at the present destination airport due to unforseen non normal events, eg, gear failures, blocked/obstructed runway, etc to allow you time to deal with situation if you cannot land immediately as was expected.
If the aircraft landing ahead of you has an accident and blocks the only available runway you may be forced to use that reserve to fly to a nearby airport where you can land or use it to conduct an off field landing.
I was reminded of this scenario yrs ago in the USA when a biz jet had a gear failure, skidded down the runway on its belly and came to rest right in the middle of the intersection of the 2 runways thus closing both of them and the airport.
Despite the long odds it can and does happen, which is why that 30 or 45 mins is kept in the tanks as last chance to help save your ass when the unexpected happens.
