Battery Backup Power: VBAT is often connected to a battery (or other backup power source) to provide power to the real-time clock (RTC) and other memory circuits that need to retain information when the main power is lost.

RTC Preservation: Microcontrollers that include a real-time clock or can keep time will use the VBAT pin to retain timekeeping capability even if the main supply $VDD$ is powered down.

Low Power: In power-saving modes, a device may draw minimal current from the VBAT to maintain essential functions.

No Battery Backup: Without a VBAT connection, any part of the circuit that requires backup power—such as a real-time clock (RTC) or certain types of memory—will lose power when the main supply, $VDD$, is turned off. This means you will lose the ability to keep time or retain certain data across power cycles.

Floating Pin: You should never leave the VBAT pin floating, even if you don't intend to use its backup functionality. A floating pin can lead to unpredictable behavior. It's generally advised to tie it to the ground (VSS) or connect it to $VDD$ to stabilize the pin's potential.

Reduced Functional Requirements: If your application does not require maintaining time or retaining specific data when powered off, you can safely ignore implementing VBAT functionality.

Design Simplicity: Not using VBAT can simplify the design, as you won't need to add a battery holder, battery circuit, or consider battery life in your system design.

Tie VBAT to VDD: Connect the VBAT pin to the main supply voltage, $VDD$. This approach ensures that even without a battery, the pin has a defined potential, preventing any erratic behavior or unexpected power drains.

Or Tie VBAT to VSS: Alternatively, connect the VBAT pin to ground, $VSS$, especially if the device documentation suggests doing so when not using this feature.

RB14: This indicates that the pin can be used as a general-purpose I/O (GPIO) pin associated with port B, bit 14. In GPIO mode, you can configure this pin as an input or output for interfacing with other digital signals in your circuit.

VBUSON1: This function suggests a specific role related to USB peripherals. Typically, "VBUSON" is used to control an external switch or circuitry when the microcontroller is involved in USB host operations. It usually indicates when the USB voltage (VBUS) should be supplied to downstream devices, essentially controlling the power to the USB devices that the microcontroller is hosting.

RTC Functionality: The VBAT pin ensures that the RTC continues to keep time even when the main MCU is powered down. This is crucial for applications where timekeeping is essential, like in data loggers or any device that needs to maintain accurate time across power cycles.

Low Power Consumption: When the MCU is powered down, the VBAT supply usually consumes very little power, allowing for a prolonged time-keeping capability without draining the battery quickly.

Battery Backup: The VBAT pin is often connected to a small battery or capacitor that can keep the RTC running for extended periods, even if the device is disconnected from its main power source.