I could imagine a scenario where gal/ft² is useful. Like with grocery store shelving figuring shelving and product stacking. If liquid storage containers are stackable then you have have more gallons per square footage of shelf space. Or of they’re not stackable, then taller containers would hold more liquid in the same shelf space than shorter containers with the same footprint.
Yeah it seems odd to represent something as a volume/area, but that is the relevant information you’re comparing and it’s intuitive how that number changes based on changes to volume as projected onto an area. Bigger number points toward a more efficient use of shelving.
Because the power draw of appliances is measured in watts, so a 60 watt light bulb when lit draws 60 watts of power over the course of one hour. So if I have roughly 100 lightbulbs at 60 watts hooked up to my house, then I’ll be using 6 kW of power each hour.
It tells us more information about the rate of use of that energy. It’s like the difference between a 2 lb sphere of uranium being exploded in a fraction of a second vs 2 lb lf uranium fuel in a reactor operating for however long that much fuel lasts for. Both contain the same amount of joules of energy at the end of the process, one just uses all of those joules in one go and the other slowly releases that energy over a longer period of time.