drastically diminish a fire’s momentum, decreasing its likelihood to spot across the firebreak, and keep going, so the firebreak can be used as a defensive line. In other words, the idea is to remove fuel from the path of a rapidly moving fire, where the head creates a powerful convective column that pulls air in from all sides, including downwind. When this flow is steady, a fire is started ahead of the main fire from a natural barrier, such as a road or river, which is sucked in and creates a wider firebreak
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Aiding in ground crew mop up, by burning off unburned areas, making it safer. Ragged edges can also be straightened, to reduce the perimeter and decrease workload.

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Reducing the time, effort, and expense of cutting fire perimeters for fires with many fingers, which is very time consuming.

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Steering a fire somewhere.

You might carry a drip torch beneath your machine and get to become an instant pyromaniac with a 20-foot flame about 20-25 feet below the cargo hook, carrying a 55-gallon fuel drum, associated pipes, wiring, etc., and a fire extinguisher to cope with hang fires, or those that start around the valve outlet when it closes. Apart from danger to the helicopter, the mixture could fall off and start unwanted fires.
The equipment weighs about 550 lbs when full, and can use straight Jet Fuel, or have it mixed with a gelling agent, which makes it cling to foliage and be more effective (I love the smell of Petrogel in the morning). There will be a propane flame for ignition. As there is a polarity-switching feature, aircraft rewiring is unnecessary and, although it demands a 25-amp circuit breaker, 30 amps is recommended.

Another way is to use polystyrene balls about an inch across, full of Potassium Permanganate, injected with Ethylene Glycol (anti-freeze) in a special dispenser and ejected from the aircraft. For maximum safety, the balls should not ignite inside ten seconds, but most take about thirty.
On larger fires, an intermediate helicopter will be dedicated for the job, on short standby and used for little else, except when prevailing conditions are not conducive to burning, and you may be assigned to other flying duties.
Always expect a Forestry Officer to be on board.
Your maximum speed is whatever you can control, after Flight Manual limitations, so burning fuel reaches the ground at about 15 kts. If you go too fast, say over 20, the mixture will be too lean and the fuel will be used up before reaching the ground. When greater than translation, fuel will recirculate to give a poor pattern, aside from damaging the equipment.
Otherwise, common-sense rules apply, such as keeping the helicopter (or the mixing area) well away from buildings, etc. on the pad, grounding everything, wear flameproof or resistant clothing and equipment, or at least cotton protective clothing, brief anything that moves, etc. Don't forget equipment spares, and to fly into wind, or you will be blinded by your own smoke.