High-grade heat can be easily turned into electricity with a turbine, or reused in an industrial process (the entire point of a nuclear reactor is to create heat in this temperature range!) Medium-grade heat can still be used for some processes or used to generate electricity, but the electricity generation will be less efficient. Low-grade heat is under 100C and is a lot harder to use. You cannot economically generate electricity from it, or use it for most industrial processes, so use cases often focus on district heating.
The problem with these low-grade-heat district heating schemes, or more broadly any use of low-grade heat, is the economics. Let's take your idea. The efficiency from sunlight to heat is indeed high (much higher than PV panel -> resistive element) but the heat generated is all low-grade heat.
So what's the root cause here, why is low grade heat usually not economic to use? It comes back to two main causes: 1) efficiency, and 2) storage. Most power is generated from turbines that use heat - a type of heat engine. Carnot efficiency is the maximum theoretical efficiency of a heat engine. Carnot efficiency is η = 1 – Tcold/Thot where temperatures are absolute temperatures (Kelvin/Rankine.) In other words, 7.7% for 50C->25C (298K->323K), 37% for 200C->25C, and 61% for 500C->25C. Note that this is _theoretical_ maximum efficiency; real world efficiency varies quite a bit - from ~1/2 to ~1/10th of Carnot efficiency at peak depending on your heat engine. The second, storage costs, are even more important. You need to insulate your warm object to keep it warm, and if your heat is low-grade, then it is spread out across a huge volume.
Back to your idea, your "paint the dirt black" idea will generate far more heat, but very low-grade and entirely non-economic to use. You will have a somewhat warm pile of dirt, but nobody really wants a somewhat warm pile of dirt. This is the same reason why you see people, logically, tearing down their high-efficiency solar water heaters to install low-efficiency solar panels.
"Use solar panels to resistively heat the dirt", on the other hand, is less efficient and generates far less heat - but it generates high-grade heat. This startup proposes to eventually sell that heat to power plants, to generate electricity directly; and if you're doing that, the temperature of the heat is critical. As you can see from the Carnot efficiency, a power plant couldn't economically do anything with a warm pile of dirt, a solar water heater, or other similar technologies. But they _can_ do something with a source of high-grade heat - namely, they can run the turbines that currently run on fossil fuels. In other words, you can solve the seasonal solar curve problem and have constant electricity production year-round, even in northerly climes.
[0] Above - very roughly - 500C, it is harder to use the waste heat efficiently, because the engineering gets a lot harder, but the theoretical maximum efficiency is higher. That's one reason why there are a lot of efforts to try to build nuclear reactors working at these higher temperatures (see: molten salt reactors.)