People have been using sub ohm coils to get higher wattages not available on the more common mods.
Now with the higher wattage mods, they no longer need to restrict them selves to sub ohm coils and can instead experiment with higher ohm coils at high wattage.
Higher wattage will produce more vapor, just as turning up the heat on the stove will boil water faster;
Although the first part of your response is correct I don't think the second part is. If you hit a 2 ohm coil with 20 watts you would need 6.3+ volts of electricity. No mod puts out that much voltage and if you hit 2 ohm coil with 6+ volts it would probably burn out very quickly.
They want higher wattage so they can run there Sub-Ohm coils on a regulated device instead of a Mech Mod. Giving them longer battery life, possibly, and not having to worry about the battery venting gas and or failing. Protecting them and the battery.
Rip Tripper did a review of the box mod, I think it was a GI2, that could go to 100 watts. He was running a .3 ohm coil at 45 watts.
That Mod was delivering 3.67+ volts to the coil. Now if you were running a .1 coil at 3.16 volts you get that 100 watts.