A scanner might reveal certain inherent weaknesses within the target host. These might be key factors in implementing an actual compromise of the target's security. In order to reap this benefit, however, you must know how to recognize the hole.
Scanners are important to Internet security because they reveal weaknesses in the network.
Whether this information is used by hackers or crackers is immaterial. If used by system administrators, scanners help strengthen security in the immediate sense. If employed by crackers, scanners also help strengthen security. This is because once a hole has been exploited, that exploitation will ultimately be discovered. Some system administrators argue that scanners work against Internet security when in the hands of crackers.