Is junk DNA completely useless?

No, junk DNA is not completely useless. 

Junk DNA refers to noncoding DNA, which, unlike coding DNA, does not provide instructions for synthesizing proteins. Earlier, scientists dismissed noncoding DNA as ‘junk’ with no known purpose and they called it junk DNA. However, further research has shown that all junk DNA is not completely useless. 

Scientists have found that some junk or noncoding DNA are essential to optimum cell functioning, particularly in regulating gene activity. These DNA contain many types of regulatory elements that play key roles in determining where and when genes are turned on and off and providing sites for transcription factors to bind. While ‘nonessential’ junk DNA can be removed without any adverse effects, removing ‘essential’ junk DNA can result in dire consequences for the organism.