Some people like them when they are on the move a lot and don't have any solar, and need to get their lithium batteries back up.
Standard light gauge truck to trailer wiring loses a lot of energy from the truck alternator to the converter/controller. When I had AGM batteries, I would get 6-9 amps from my truck, and now with lithium batteries, I only get about 3 amps from the truck. I'm guessing that's from lithium not have the gradual voltage drop that acid batteries have.
With DC-DC, the package has much heavier wiring along with a unit to control it.
Once in a while I would like one but usually don't need it. I have 1000 watts of solar with 400 amps of lithium.
JK