Not all USB-C ports on laptops support charging. For example, the Razer Blade 15 FAQs state that it does not support USB-C charging, although that limitation may only exist due to the laptop requiring more power than the USB-PD standard can provide (>100W).
Surface Laptops 1&2 seem to come with a 39W charger, so it should be well withing USB-PD standard.
I'm looking for confirmation that USB-C charging though USB-PD is officially supported specifically on the Surface Laptop 3 and that the port is not for only data.