Why use it? Per the instructions: "Who Must File
If you’re filing a 2019 Form 1120-S through
an ISP and/or transmitter and you’re not
using an ERO, you must file Form 8453-S
with your electronically filed return. An ERO
can use either Form 8453-S or Form
8879-S to obtain authorization to file the
corporation’s Form 1120-S."

"When and Where To File
File Form 8453-S with the corporation’s
electronically filed income tax return. Use a
scanner to create a Portable Document
Format (PDF) file of the completed form.
Your tax preparation software will allow
you to transmit this PDF file with the return"