From the announcement:
There are benefits to placing items prior to ignition compared with using them to stoke the fire after ignition. Placing combustibles prior to ignition gives a 1.25x bonus to their spread, while stoking with combustibles after ignition gives a 1.25x bonus to their intensity.
Intensity and spread do both grow on their own from momentum; it might be that the "optimal" play here is to douse waaay more before you light up? That would consistently use a large amount of resources, whereas lighting early and adding more fuel afterwards is riskier (hosp/jail possibility) but reduces the total resources you need to commit.