Scan modes are not just for testing the flops, it is testing whether the design ( both sequential and combinational) is manufactured as intended. During manufacturing process, there is so much process variation and manufacturing defects. These defects may cause electrical faults like shorts to power/ground and others which can be represented as logical fault of stuck at 0 and stuck at 1. These faults may modify the original intended logic and cause chip to malfunction. During Scan shift mode, you shift the scan patterns ( which helps in determining faults) into the chip ,and during scan capture mode, you capture the combinational logic output to scannable flops and compare the value with expected value. If it doesn't match we know there is a fault in the chip.