F-Secure BlackLight is able to correctly ignore non-malicious objects and alerts only on real rootkits, which makes it useful even for users without technical knowledge. It is also able to deal correctly with files that have been modified during the scanning process. This makes it possible to use it in the background without interrupting normal work.

Rootkits for Windows work in a different way and are typically used to hide malicious software from for example an antivirus scanner. Rootkits are typically not malicious by themselves but are used for malicious purposes by viruses, worms, backdoors and spyware. A virus c****ined with a rootkit produces what was known as full stealth viruses in the MS-DOS environment.
The rootkit itself does typically not cause deliberate damage. Its purpose is to hide software. But rootkits are used to hide malicious code. A virus, worm, backdoor or spyware program could remain active and undetected in a system for a long time if it uses a rootkit.
The malware may remain undetected even if the computer is protected with state-of-the-art antivirus. And the antivirus can't remove something that it can't see. The threat from modern malware c****ined with rootkits is very similar to full stealth viruses that caused a lot of headache during the MS-DOS era. All this makes rootkits a significant threat.