What is rating scan for?

It seems to be the same as quick scan except it searches for known \ unknown files instead of viruses. Yeah it may be a quick way to build trusted files list, but that list will build itself anyway as soon as you start trusted apps.
Is it just a gimmick to build trusted list after CIS install or is there other uses?

It does the same scan as “quick scan” (i.e areas) but asks the cloud for file reputation.

More information on Quick vs. Rating Scan and COMODO’s cloud service.

Running a Quick Scan

Running a Ratings Scan

How the COMODO Cloud works (Direct from the help site)

Cloud Based Behavior Analysis
Comodo Internet Security features a cloud based analysis of unrecognized files, in which any file that is not recognized and not in Comodo’s white-list will be sent to Comodo Instant Malware Analysis (CIMA) server for behavior analysis. Each file is executed in a virtual environment on Comodo servers and tested to determine whether it contains any malicious code. The results will be sent back to your computer in around 15 minutes.