ADXL354 Power Sequencing Sources and LDO's
The V1P8DIG and V1P8ANA provide 1.8V to digital and analog sections respectively. They are internal LDO's that drop the main VCC to around 1.8V, which the IC needs to function. If you already have 1.8V then the LDO's don't work, so you'll bypass the LDO's. My guess is the LDO's are provided because most people use 3.3V for Vcc, you would be less likely to use this accelerator in a product if you had to provide your own 1.8V LDO which adds cost and space (especially since most acellerometers go into products that have tight weight/space requirements like cellphones and drones). 


  The LDO regulators are operational when VSUPPLY is between 2.25 V and
  3.6 V. V1P8ANA and V1P8DIG are the regulator outputs in this mode. Alternatively, when tying VSUPPLY to VSS, V1P8ANA and V1P8DIG are supply voltage inputs with a 1.62 V to 1.98 V range.


Source: https://www.analog.com/media/en/technical-documentation/data-sheets/adxl354_355.pdf