In a car what is the point in an inverter, converting DC to AC, for powering up a phone or laptop when the battery of such devices charges with DC?
It provides an AC output which you can run a single household appliance with. Each item such a laptop, phone etc. will have different DC input requirements. If you wanted to charge a modern laptop or phone and the car has a USB C port, that would be the more efficient way of doing it.
Take a look at all of your devices, the laptop, tablet, phone, I bet they all have a different charging lead and most likely different charging voltages. The one thing they all have in common is the charger runs off 230V AC, I rest my case.
For bigger items like fridges, washers, ASHP etc, the 240v AC is converted to DC using simple rectification and using an inverter converts it back to AC. Only now that AC is at a different voltage and frequency to the input and can be varied to suit the motor speed and its load at that moment. Sounds convoluted but modern power electronics make it easy to control motors via an inverter.
To decrease a DC voltage you can turn excess into heat, but clearly wasting energy, but to increase voltage not so easy, either a motor drives a dynamo with rotary converter, or use some static inverter, in the old days we used a syncrous vibrator that connected to a transformer, the vibrator made the DC into AC. The big change was the front loading VCR, needed to get a DC stable voltage without generating heat, and the switch mode power supply was born. Does not matter what name, pulse width modulation, or switch mode, it have transformed how we change voltages, even with AC at 50 Hz we turn it into DC, then back to AC in kHz range which means can use a smaller transformer the back to DC and then back to AC at 50 Hz. The first transistor was field effect, but it was hard to perfect so we started with the bi-polar but now nearly all FET's and FET's produce less heat, so can be put into smaller packages. So the whole thing revolves around producing less heat, and switch mode power supplies produce less heat to other options.
Plus, SMPS only has a small wire wound inductor instead of a massive transformer used in linear power supplies. These days anything wirewound is expensive relative to any semiconductor device.