Dial face gages are also known as analog pressure gages. They use a mechanical gear/needle and a bourdon tube to indicate pressure.
The most important reason for a dial face gauge on an automated system is for safety – this allows the operator to determine if there is pressure in a vessel or a portion of the system without the need for power – i.e. during a power outage.
The second reason for a dial face gauge on an automated system is for redundancy – this allows the operator to compare the analog readings on the gage to the digital readings on the automation system. Both mechanical and electrical instruments can fail. Having redundant indications allows the operator to identify an issue and quickly correct it. Without redundancy there is no way for an operator to know there is an issue and thus can operate with incorrect readings!
A note on accuracy – automation platforms with digital indications will always be able to display a significantly higher resolution and responsiveness than a typical analog dial face gage found on CO2 systems. There are high accuracy pressure gages that are typically used for calibration, but are not typically used on CO2 systems.
I also addressed a similar question in a separate post that can be found here: https://cannabis.expertcallingnetwork.com/ask/i-understand-that-fully-automated-machine-sensors-can-get-gummed-up-with-oil-causing-machines-to-fail-or-for-readings-to-be