I'm putting a Yunshis PLS-K-40 / SK40 LiDAR on a Raspberry Pi and I'd rather wire it to the Pi's UART pins than hang a USB adapter off it. Is the adapter actually required, or is it just for convenience?
Not required. If the module provides a standard UART TTL interface, you can connect it directly to the Pi's UART. Confirm the UART voltage level (3.3V TTL), the pin definitions (VCC, GND, TX, RX) and the communication parameters (baud rate, protocol) first.
For the PLS-K-40 / SK40 UART TTL versions:
The TTL-to-USB adapter is mainly for PC testing, debugging and running the manufacturer's tools. Direct UART is the usual choice for embedded systems, Raspberry Pi and custom controllers once the level and pinout are confirmed.