Being a first generation IACS user, I was told back then......

First check - By the airport authority that you are allowed restricted access into airside by matching boarding pass and passport. This is done by the airport security (Cisco).

Second check - is the smart card immigration to verify your identity (card/thumb print) and to log out as having departed Singapore.

Third check - is immigration program compliance, that a travelling passenger into/out of Singapore must have all their travel documents (IC, Passport, Work Permit, Employment Pass, Dependent Pass, Student Pass etc carried & valid).

Back in the days, ICAS was voluntary........

This is no different from using Canada/US NEXUS & US Global Entry, but their document verification is random as opposed to being mandatory at all times. Actually, less random crossing into the US as they've been asking me for passport in addition to my NEXUS card all the time.