If you read the ICA website it says
"To enable us to assess your application holistically, it is important that your employer provides the information requested in Annex A. "
https://www.ica.gov.sg/docs/default-sou ... 284ffa.pdf
While i doesn't tell us if it's mandatory or not, it allows ICA to make a clearer determination of your employer.
For example, saying "IT" in your application could mean everything from installing IoT items at people' homes to being a researcher in quantum computing. Or could be "Challenger" shop helpdesk (admittedly it would probably be under retail) or an Amazon AWS engineer.
Anecdotal reporting here suggest that you may not have to provide it, but why not make it easier for ICA by supporting your application with more information about your company.
If you don't provide it, they probably have a heap of more applicants who are willing to provide their Annex A.