Most backpacking stoves aren't pressure-regulated. As their canister pressure drops, their output also declines. A stove with a regulator, in contrast, isn't hindered by that drop in canister pressure (it's designed to operate optimally at a very low pressure, and the regulator maintains that pressure). Because of this, you enjoy fast boil times regardless of what's happening to the canister.
In other words, the regulator keeps your stove cranking at full output across a wider range of conditions and scenarios—hot days, colder days, full canister, low canister and everything in between. It also means that when your canister is low, the stove isn't struggling. This saves you fuel. Therefore, a pressure regulator also boosts the efficiency of your stove, which means you can carry less fuel. If you really want to geek out, here's what that looks like numbers: An unregulated stove is designed to deliver its max output when the ambient air is 70°F. At this temperature, the canister pressure is 45 psi. When the temp drops to 50°F, canister pressure drops to 30 psi. As you run your stove, your canister cools further, easily dropping to 22 psi. And on that cool fall morning (40°F), your stove may only be putting out less than 60% of its power.
In contrast, MSR’s pressure-regulated stoves are designed to run at full output at just 15 psi (Reactor) and 16.5 psi (WindBurner, PocketRocket Deluxe, and Switch). The regulator ensures the stove receives that precise pressure regardless of the real pressure inside the canister. It’s only when your canister is nearly empty or the conditions are extremely cold that the pressure will finally drop below the limit of the pressure regulator and you’ll notice a performance drop. All this means that with a well-regulated stove, you'll enjoy high stove performance through a far greater range of conditions. And your fourth pot of water will boil just as fast as your first.
Check out our blog post for more detailed information on this topic:
https://cascadedesigns.com/blogs/msr-gear-guides/technology-stove-pressure-regulators-work