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?
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.