The most ubiquitous use of the FT231X is as a USB-to-Serial adapter dongle. It is frequently used to program Arduino-compatible boards (bootloader flashing) and debug ESP32, STM32, and ARM Cortex-M microcontrollers via their UART bootloaders or SWD interfaces (when combined with specific software).
The FT231X USB UART Bridge: Architecture, Implementation, and Application in Modern Embedded Systems ft231x usb uart
The evolution of the personal computer has seen the obsolescence of the DB-9 serial connector, previously the standard for device programming and debugging. However, the underlying UART protocol remains the dominant standard for low-level microcontroller (MCU) communication due to its simplicity and low overhead. The most ubiquitous use of the FT231X is